UltraSonic Radar

This is a very interesting project with many practical applications in security and alarm systems for homes, shops and cars. It consists of a set of ultrasonic receiver and transmitter which operate at the same frequency. When something moves in the area covered by the circuit the circuits fine balance is disturbed and the alarm is triggered. The circuit is very sensitive and can be adjusted to reset itself automatically or to stay triggered till it is reset manually after an alarm.


How it Works

As it has already been stated the circuit consists of an ultrasonic transmitter and a receiver both of which work at the same frequency. They use ultrasonic piezoelectric transducers as output and input devices respectively and their frequency of operation is determined by the particular devices in use.

The transmitter is built around two NAND gates of the four found in IC3 which are used here wired as inverters and in the particular circuit they form a multivibrator the output of which drives the transducer. The trimmer P2 adjusts the output frequency of the transmitter and for greater efficiency it should be made the same as the frequency of resonance of the transducers in use. The receiver similarly uses a transducer to receive the signals that are reflected back to it the output of which is amplified by the transistor TR3, and IC1 which is a 741 op-amp. The output of IC1 is taken to the non inverting input of IC2 the amplification factor of which is adjusted by means of P1. The circuit is adjusted in such a way as to stay in balance as long the same as the output frequency of the transmitter. If there is some movement in the area covered by the ultrasonic emission the signal

that is reflected back to the receiver becomes distorted and the circuit is thrown out of balance. The output of IC2 changes abruptly and the Schmitt trigger circuit which is built around the remaining two gates in IC3 is triggered. This drives the output transistors TR1,2 which in turn give a signal to the alarm system or if there is a relay connected to the circuit, in series with the collector of TR1, it becomes activated. The circuit works from 9-12 VDC and can be used with batteries or a power supply.


Construction

First of all let us consider a few basics in building electronic circuits on a printed circuit board. The board is made of a thin insulating material clad with a thin layer of conductive copper that is shaped in such a way as to form the necessary conductors between the various components of the circuit. The use of a properly designed printed circuit board is very desirable as it speeds construction up considerably and reduces the possibility of making errors. Smart Kit boards also come pre-drilled and with the outline of the components and their identification printed on the component side to make construction easier. To protect the board during storage from oxidation and assure it gets to you in perfect condition the copper is tinned during manufacturing and covered with a special varnish that protects it from getting oxidised and also makes soldering easier. Soldering the components to the board is the only way to build your circuit and from the way you do it depends greatly your success or failure. This work is not very difficult and if you stick to a few rules you should have no problems. The soldering iron that you use must be light and its power should not exceed the 25 Watts. The tip should be fine and must be kept clean at all times. For this purpose come very handy specially made sponges that are kept wet and from time to time you can wipe the hot tip on them to remove all the residues that tend to accumulate on it. DO NOT file or sandpaper a dirty or worn out tip. If the tip cannot be cleaned, replace it. There are many different types of solder in the market and you should choose a good quality one that contains the necessary flux in its core, to assure a perfect joint every time. DO NOT use soldering flux apart from that which is already included in your solder. Too much flux can cause many problems and is one of the main causes of circuit malfunction. If nevertheless you have to use extra flux, as it is the case when you have to tin copper wires, clean it very thoroughly after you finish your work. In order to solder a component correctly you should do the following:

• Clean the component leads with a small piece of emery paper.
• Bend them at the correct distance from the component�s body and insert the component in its place on the board.
• You may find sometimes a component with heavier gauge leads than usual, that are too thick to enter in the holes of the p.c. board.
• In this case use a mini drill to enlarge the holes slightly. Do not make the holes too large as this is going to make soldering difficult afterwards.
• Take the hot iron and place its tip on the component lead while holding the end of the solder wire at the point where the lead emerges from the board. The iron tip must touch the lead slightly above the p.c. board.
• When the solder starts to melt and flow wait till it covers evenly the area around the hole and the flux boils and gets out from underneath the solder. The whole operation should not take more than 5 seconds. Remove the iron and allow the solder to cool naturally without blowing on it or moving the component. If everything was done properly the surface of the joint must have a bright metallic finish and its edges should be smoothly ended on the component lead and the board track. If the solder looks dull, cracked,or has the shape of a blob then you have made a dry joint and you should remove the solder (with a pump, or a solder wick) and redo it.
• Take care not to overheat the tracks as it is very easy to lift them from the board and break them.
• When you are soldering a sensitive component it is good practice to hold the lead from the component side of the board with a pair of long-nose pliers to divert any heat that could possibly damage the component.
• Make sure that you do not use more solder than it is necessary as you are running the risk of short-circuiting adjacent tracks on the board, especially if they are very close together.
• When you finish your work cut off the excess of the component leads and clean the board thoroughly with a suitable solvent to remove all flux residues that may still remain on it.
• There are quite a few components in the circuit and you should be careful to avoid mistakes that will be difficult to trace and repair afterwards. Solder first the pins and the IC sockets and then following if that is possible the parts list the resistors the trimmers and the capacitors paying particular attention to the correct orientation of the electrolytic.

Solder then the transistors and the diodes taking care not to overheat them during soldering. The transducers should be positioned in such a way as they do not affect each other directly because this will reduce the efficiency of the circuit. When you finish soldering, check your work to make sure that you have done everything properly, and then insert the IC�s in their sockets paying attention to their correct orientation and handling IC3 with great care as it is of the CMOS type and can be damaged quite easily by static discharges. Do not take it out of its aluminium foil wrapper till it is time to insert it in its socket, ground the board and your body to discharge static electricity and then insert the IC carefully in its socket. In the kit you will find a LED and a resistor of 560 � which will help you to make the necessary adjustments to the circuit. Connect the resistor in series with the LED and then connect them between point 9 of the circuit and the positive supply rail (point 1).

Connect the power supply across points 1 (+) and 2 (-) of the p.c. board and put P1 at roughly its middle position. Turn then P2 slowly till the LED lights when you move your fingers slightly in front of the transducers. If you have a frequency counter then you can make a much more accurate adjustment of the circuit. Connect the frequency counter across the transducer and adjust P2 till the frequency of the oscillator is exactly the same as the resonant frequency of the transducer. Adjust then P1 for maximum sensitivity. Connecting together pins 7 & 8 on the p.c. board will make the circuit to stay triggered till it is manually reset after an alarm. This can be very useful if you want to know that there was an attempt to enter in the place which are protected by the radar.

Componets:

R1 = 180 KOhm
R2 = 12 KOhm
R3, 8 = 47 KOhm
R4 = 3,9 KOhm
R5, 6, 16 = 10 KOhm
R7, 10, 12, 14, 17 = 100 KΩ
R9, 11 = 1 MOhm
R13, 15 = 3,3 KOhm
C1, 6 = 10uF/16V
C2 = 47uF/16V
C3 = 4,7 pF
C4, 7 = 1 nF
C5 = 10nF
C8, 11 = 4,7 uF/16
C9 = 22uF/16V
C10 = 100 nF
C12 = 2,2 uF/16V
C13 = 3,3nF
C14 = 47nF
TR1, 2, 3 = BC547 , BC548
P1 = 10 KOhm trimmer
P2 = 47 KOhm trimmer
IC1, 2 = 741 OP-AMP
IC3 = 4093 C-MOS
R = TRANSDUCER 40KHz
T = TRANSDUCER 40KHz
D1, 2, 3, 4 = 1N4148

Automatic Room Lights

n ordinary automatic room power control circuit has only one light sensor. So when a person enters the room it gets one pulse and the lights come ‘on.’ When the person goes out it gets another pulse and the lights go ‘off.’ But what happens when two persons enter the room, one after the other? It gets two pulses and the lights remain in ‘off’ state. The circuit described here overcomes the above-mentioned problem. It has a small memory which enables it to automatically switch ‘on’ and switch ‘off’ the lights in a desired fashion. The circuit uses two LDRs which are placed one after another (separated by a distance of say half a metre) so that they may separately sense a person going into the room or coming out of the room. Outputs of the two LDR sensors, after processing, are used in conjunction with a bicolour LED in such a fashion that when a person gets into the room it emits green light and when a person goes out of the room it emits red light, and vice versa.



These outputs are simultaneously applied to two counters. One of the counters will count as +1, +2, +3 etc when persons are getting into the room and the other will count as -1, -2, -3 etc when persons are getting out of the room. These counters make use of Johnson decade counter CD4017 ICs. The next stage comprises two logic ICs which can combine the outputs of the two counters and determine if there is any person still left in the room or not. Since in the circuit LDRs have been used, care should be taken to protect them from ambient light. If desired, one may use readily available IR sensor modules to replace the LDRs.

The sensors are installed in such a way that when a person enters or leaves the room, he intercepts the light falling on them sequentially—one after the other. When a person enters the room, first he would obstruct the light falling on LDR1, followed by that falling on LDR2. When a person leaves the room it will be the other way round. In the normal case light keeps falling on both the LDRs, and as such their resistance is low (about 5 kilo-ohms). As a result, pin 2 of both timers (IC1 and IC2), which have been configured as monostable flip-flops, are held near the supply voltage (+9V).

When the light falling on the LDRs is obstructed, their resistance becomes very high and pin 2 voltages drop to near ground potential, thereby triggering the flip-flops. Capacitors across pin 2 and ground have been added to avoid false triggering due to electrical noise. When a person enters the room, LDR1 is triggered first and it results in triggering of monostable IC1.

The short output pulse immediately charges up capacitor C5, forward biasing transistor pair T1-T2. But at this instant the collectors of transistors T1 and T2 are in high impedance state as IC2 pin 3 is at low potential and diode D4 is not conducting. But when the same person passes LDR2, IC2 monostable flip-flop is triggered. Its pin 3 goes high and this potential is coupled to transistor pair T1-T2 via diode D4. As a result transistor pair T1-T2 conducts because capacitor C5 retains the charge for some time as its discharge time is controlled by resistor R5 (and R7 to an extent).

Thus green LED portion of bi-colour LED is lit momentarily. The same output is also coupled to IC3 for which it acts as a clock. With entry of each person IC3 output (high state) keeps advancing. At this stage transistor pair T3-T4 cannot conduct because output pin 3 of IC1 is no longer positive as its output pulse duration is quite short and hence transistor collectors are in high impedance state. When persons leave the room, LDR2 is triggered first followed by LDR1.

Since the bottom half portion of circuit is identical to top half, this time with the departure of each person red portion of bi-colour LED is lit momentarily and output of IC4 advances in the same fashion as in case of IC3. The outputs of IC3 and those of IC4 (after inversion by inverter gates N1 through N4) are ANDed by AND gates (A1 through A4) are then wire ORed (using diodes D5 through D8). The net effect is that when persons are entering, the output of at least one of the AND gates is high, causing transistor T5 to conduct and energise relay RL1.

The bulb connected to the supply via N/O contact of relay RL1 also lights up. When persons are leaving the room, and till all the persons who entered the room have left, the wired OR output continues to remain high, i.e. the bulb continues to remains ‘on,’ until all persons who entered the room have left. The maximum number of persons that this circuit can handle is limited to four since on receipt of fifth clock pulse the counters are reset.

The capacity of the circuit can be easily extended for up to nine persons by removing the connection of pin 1 from reset pin (15) and utilising Q1 to Q9 outputs of CD4017 counters. Additional inverters, AND gates and diodes will, however, be required.

Battery Low Voltage Beeper

This electronic circuit is an alarm circuit for low battery condition. It provides an audible and visual low voltage warning for 12V battery powered devices. When the battery voltage is above the set point (typically 11V), the circuit is idle. If the battery voltage should fall below the set point, the LED will light and the speaker will emit a periodic beeping sound to warn of the impending loss of power. The circuit was designed for monitoring solar systems, but it could also be useful for automotive and other 12V applications.



How it works

U2 provides a 5V regulated voltage reference. U1 is wired as a comparator, it compares the fixed 5V regulated voltage to the voltage on the wiper of VR1, that is proportional to the 12V supply. When the supply drops below the set point, the output of U1 goes low, turning on Q1 and powering the beeper and the LED.




The beeper consists of U4, a tone generator, and U3, a low duty cycle pulse generator. The tone can be changed by adjusting R7, the beep rate can be changed by adjusting R5. A small amount of hysteresis is provided by R1 and the current through LED1 and the beeper, this separates the on and off points for the circuit.

U2 provides a 5V regulated voltage reference. U1 is wired as a comparator, it compares the fixed 5V regulated voltage to the voltage on the wiper of VR1, that is proportional to the 12V supply. When the supply drops below the set point, the output of U1 goes low, turning on Q1 and powering the beeper and the LED.

The beeper consists of U4, a tone generator, and U3, a low duty cycle pulse generator. The tone can be changed by adjusting R7, the beep rate can be changed by adjusting R5. A small amount of hysteresis is provided by R1 and the current through LED1 and the beeper, this separates the on and off points for the circuit.

Use of Battery Low Voltage Beeper

Connect the circuit to the 12V source that you wish to monitor. Turn S1 on, if the battery voltage is above the set point, nothing should happen.

As the battery voltage drops below the set point, the LED will light and a periodic beeping will come from the speaker. If the beeping becomes annoying, turn off S1. Be sure to charge the battery soon, excessive discharging will shorten the life of most rechargeable batteries.

Water Detector With Sump/Bilge Pump Controller

This moisture detector with pump controller is built around the special purpose LM1830 IC. The LM1830 is designed to detect moisture by passing an AC current through a set of probes. An internal comparator compares the resistance of the probes to an internal reference. When the resistance between the probes is low (indicating the presence of water or other conductive liquid) then the IC triggers it's output. An AC signal avoids electroplating and corrosion problems that are common when DC is used. To provide an adjustable delay hysteresis to avoid cycling the pump, a timer based around the 555 is used a range of about 5 seconds to two minutes. In this way the pump will stay active for a set amount of time even if the fluid level is below that of the probes.



Parts

• R1 ------ 1 470 Ohm 1/4 W Resistor
• R2, R4 ---- 1 10K 1/4 W Resistor
• R3 ------- 1 6.8K 1/4 W Resistor
• R5 ------- 1 1 Meg Linear Taper Pot
• R6 ------- 1 51K 1/4 W Resistor
• C1 ------- 1 470uF 35V Electrolytic Capacitor
• C2 ------- 1 0.001uF Ceramic Disc Capacitor
• C3 ------- 1 0.05 uF Ceramic Disc Capacitor
• C4 ------- 1 20uF 35V Electrolytic Capacitor
• C5 ------- 1 6.8uF 35V Electrolytic Capacitor
• C6, C8 ---- 2 0.01uF Ceramic Disc Capacitor
• C7 ------- 1 100uF 35V Electrolytic Capacitor
• U1 ------- 1 LM1830 Fluid Detection IC
• U2 ------- 1 555 Timer
• Q1 ------- 1 2N5305 NPN Power Transistor
• D1, D2 ---- 2 1N4148 Signal Diode
• D3 ------ 1 1N4002 Rectifier Diode
• K1 ------- 1 Relay With 12V Coil (See Notes)
• S1 ------- 1 SPST Switch
• PROBES ---- 1 Stainless Steel Probes (See Notes)
• MISC ----- 1 Case, Knob, Board, Wire, Sockets For U1 and U2

Notes

• The probes should be made of stainless steel and mounted in a non-conductive fixture. Stainless steel nails, bolts or welding rod will make ideal probes. Good non-conductive materials are Lexan, plexiglass or plastic. The probes are to be placed at the liquid level at which you wish the pump to turn on.
• K1 should be chosen according to the requirements of the pump. If you are running a 12V pump then common automotive relays will work fine. If you are running a mains powered pump then you will need to make sure the contacts are rated accordingly.
• S1 provides a manual pump switch.
• R5 adjusts the on-time of the pump once moisture has been detected. It is adjustable from approximately 5 seconds to approximately 2 minutes.
• The probes should be cleaned periodically to assure reliable operation.

Air Flow Detector

This simple circuit uses an incandescent lamp to detect airflow. With the filament exposed to air, a constant current source is used to slightly heat the filament. As it is heated, the resistance increases. As air flows over the filament it cools down, thus lowering it's resistance. A comparator is used to detect this difference and light an LED. With a few changes, the circuit can be connected to a meter or ADC to provide an estimation on the amount of air flow.

Parts

• R1 ---- 1 100 Ohm 1/4W Resistor
• R2 ---- 1 470 Ohm 1/4W Resistor
• R3 ---- 1 10k 1/4W Resistor
• R4 ---- 1 100K 1/4W Resistor
• R5 ---- 1 1K 1/4W Resistor
• C1 ---- 1 47uF Electrolytic Capacitor
• U1 ---- 1 78L05 Voltage Regulator
• U2 ---- 1 LM339 Op Amp
• L1 ---- 1 #47 Incandescent lamp with glass removed (See "Notes")
• D1 ---- 1 LED
• MISC --- 1 Board, Wire, Sockets for ICs, etc.

Notes

• The glass will have to be removed from L1 without breaking the filament. Wrap the glass in masking tape and it in a vise. Slowly crank down until the glass breaks, then remove the bulb and carefully peel back the tape. If the filament has broken, you will need another lamp.

Park-Aid

Three LEDs signal bumper-barrier distance
Infra-red operation, indoor use

This circuit was designed as an aid in parking the car near the garage wall when backing up. LED D7 illuminates when bumper-wall distance is about 20 cm., D7+D6 illuminate at about 10 cm. and D7+D6+D5 at about 6 cm. In this manner you are alerted when approaching too close to the wall.

All distances mentioned before can vary, depending on infra-red transmitting and receiving LEDs used and are mostly affected by the color of the reflecting surface. Black surfaces lower greatly the device sensitivity. Obviously, you can use this circuit in other applications like liquids level detection, proximity devices etc.


Parts:

• R1_____________10K 1/4W Resistor
• R2,R5,R6,R9_____1K 1/4W Resistors
• R3_____________33R 1/4W Resistor
• R4,R11__________1M 1/4W Resistors
• R7______________4K7 1/4W Resistor
• R8______________1K5 1/4W Resistor
• R10,R12-R14_____1K 1/4W Resistors
• C1,C4___________1µF 63V Electrolytic or Polyester Capacitors
• C2_____________47pF 63V Ceramic Capacitor
• C3,C5_________100µF 25V Electrolytic Capacitors
• D1_____________Infra-red LED
• D2_____________Infra-red Photo Diode (see Notes)
• D3,D4________1N4148 75V 150mA Diodes
• D5-7___________LEDs (Any color and size)
• IC1_____________555 Timer IC
• IC2___________LM324 Low Power Quad Op-amp
• IC3____________7812 12V 1A Positive voltage regulator IC

Circuit operation:

IC1 forms an oscillator driving the infra-red LED by means of 0.8mSec. pulses at 120Hz frequency and about 300mA peak current. D1 & D2 are placed facing the car on the same line, a couple of centimeters apart, on a short breadboard strip fastened to the wall. D2 picks-up the infra-red beam generated by D1 and reflected by the surface placed in front of it. The signal is amplified by IC2A and peak detected by D4 & C4. Diode D3, with R5 & R6, compensates for the forward diode drop of D4. A DC voltage proportional to the distance of the reflecting object and D1 & D2 feeds the inverting inputs of three voltage comparators. These comparators switch on and off the LEDs, referring to voltages at their non-inverting inputs set by the voltage divider resistor chain R7-R10.

Circuit modification:

A circuit modification featuring an audible alert instead of the visual one is available here: Park-Aid Modification

Notes:

• Power supply must be regulated (hence the use of IC3) for precise reference voltages. The circuit can be fed by a commercial wall plug-in adapter, having a DC output voltage in the range 12-24V.
• Current drawing: LEDs off 40mA; all LEDs on 60mA @ 12V DC supply.
• The infra-red Photo Diode D2, should be of the type incorporating an optical sunlight filter: these components appear in black plastic cases. Some of them resemble TO92 transistors: in this case, please note that the sensitive surface is the curved, not the flat one.
• Avoid sun or artificial light hitting directly D1 & D2.
• If your car has black bumpers, you can line-up the infra-red diodes with the (mostly white) license or number plate.
• It is wiser to place all the circuitry near the infra-red LEDs in a small box. The 3 signaling LEDs can be placed far from the main box at an height making them well visible by the car driver.
• The best setup is obtained bringing D2 nearer to D1 (without a reflecting object) until D5 illuminates; then moving it a bit until D5 is clearly off. Usually D1-D2 optimum distance lies in the range 1.5-3 cm.
• If you are needing a simpler circuit of this kind driving a LED or a relay, click Infra-red Level Detector

Speed-limit Alert

Wireless portable unit
Adaptable with most internal combustion engine vehicles

This circuit has been designed to alert the vehicle driver that he/she has reached the maximum fixed speed limit (i.e. in a motorway). It eliminates the necessity of looking at the tachometer and to be distracted from driving. There is a strict relation between engine's RPM and vehicle speed, so this device controls RPM, starting to beep and flashing a LED once per second, when maximum fixed speed is reached. Its outstanding feature lies in the fact that no connection is required from circuit to engine.


Circuit operation:

IC1 forms a differential amplifier for the electromagnetic pulses generated by the engine sparking-plugs, picked-up by sensor coil L1. IC2A further amplifies the pulses and IC2B to IC2F inverters provide clean pulse squaring. The monostable multivibrator IC3A is used as a frequency discriminator, its pin 6 going firmly high when speed limit (settled by R11) is reached. IC3B, the transistors and associate components provide timings for the signaling part, formed by LED D5 and piezo sounder BZ1. D3 introduces a small amount of hysteresis.


Parts:

• R1,R2,R19_______1K 1/4W Resistors
• R3-R6,R13,R17_100K 1/4W Resistors
• R7,R15__________1M 1/4W Resistors
• R8_____________50K 1/2W Trimmer Cermet
• R9____________470R 1/4W Resistor
• R10___________470K 1/4W Resistor
• R11___________100K 1/2W Trimmer Cermet (see notes)
• R12___________220K 1/4W Resistor (see notes)
• R14,R16________68K 1/4W Resistors
• R18____________22K 1/4W Resistor
• R20___________150R 1/4W Resistor (see notes)
• C1,C7_________100µF 25V Electrolytic Capacitors
• C2,C3_________330nF 63V Polyester Capacitors
• C4-C6___________4µ7 25V Electrolytic Capacitors
• D1,D5______Red LEDs 3 or 5mm.
• D2,D3________1N4148 75V 150mA Diodes
• D4________BZX79C7V5 7.5V 500mW Zener Diode
• IC1__________CA3140 or TL061 Op-amp IC
• IC2____________4069 Hex Inverter IC
• IC3____________4098 or 4528 Dual Monostable Multivibrator IC
• Q1,Q2_________BC238 25V 100mA NPN Transistors
• L1_____________10mH miniature Inductor (see notes)
• BZ1___________Piezo sounder (incorporating 3KHz oscillator)
• SW1____________SPST Slider Switch
• B1_______________9V PP3 Battery (see notes)
• Clip for PP3 Battery

Notes:

• D1 is necessary at set-up to monitor the sparking-plugs emission, thus allowing to find easily the best placement for the device on the dashboard or close to it. After the setting is done, D1 & R9 can be omitted or switched-off, with battery savings.
• During the preceding operation R8 must be adjusted for better results. The best setting of this trimmer is usually obtained when its value lies between 10 and 20K.
• You must do this first setting when the engine is on but the vehicle is stationary.
• The final simplest setting can be made with the help of a second person. Drive the vehicle and reach the speed needed. The helper must adjust the trimmer R11 until the device operates the beeper and D5. Reducing vehicle's speed the beep must stop.
• L1 can be a 10mH small inductor usually sold in the form of a tiny rectangular plastic box. If you need an higher sensitivity you can build a special coil, winding 130 to 150 turns of 0.2 mm. enameled wire on a 5 cm. diameter former (e.g. a can). Extract the coil from the former and tape it with insulating tape making thus a stand-alone coil.
• Current drawing is about 10mA. If you intend to use the car 12V battery, you can connect the device to the lighter socket. In this case R20 must be 330R.
• Depending on the engine's cylinders number, R11 can be unable to set the device properly. In some cases you must use R11=200K and R12=100K or less.
• If you need to set-up the device on the bench, a sine or square wave variable generator is required.
• To calculate the frequency relation to RPM in a four strokes engine you can use the following formula:

• Hz= (Number of cylinders * RPM) / 120.
• For a two strokes engine the formula is: Hz= (Number of cylinders * RPM) / 60.
• Thus, for a car with a four strokes engine and four cylinders the resulting frequency @ 3000 RPM is 100Hz.
• Temporarily disconnect C2 from IC1 pin 6. Connect the generator output across C2 and Ground. Set the generator frequency to e.g. 100Hz and trim R11 until you will hear the beeps and LED D5 will start flashing. Reducing the frequency to 99 or 98 Hz, beeping and flashing must stop.
• Please note that this circuit is not suited to Diesel engines.

Three-Level Audio Power Indicator

Battery-operated 3 LED display
Simply connect it to loudspeaker output

Circuit operation:

This circuit is intended to indicate the power output level of any audio amplifier. It is simple, portable, and displays three power levels that can be set to any desired value. For a standard HiFi stereo power amplifier like the 25W MosFet Audio Amplifier described in these pages, the power output values suggested are as follows:

D5 illuminates at 2W
D4 illuminates at 12.5W
D3 illuminates at 24.5W

The above values were chosen for easy setup, but other settings are possible.
IC1A is the input buffer, feeding 3 voltage comparators and LEDs drivers by means of a variable dc voltage obtained by R5 and C4 smoothing action. In order to achieve setting stability, the supply of IC1 and trimmers R6 & R7 is reduced and clamped to 5.1V by Zener diode D1.

Parts:

• R1__________100K 1/4W Resistor
• R2___________50K 1/2W Trimmer Cermet
• R3__________330K 1/4W Resistor
• R4____________1M2 1/4W Resistor
• R5__________470K 1/4W Resistor
• R6,R7_______500K 1/2W Trimmers Cermet
• R8____________1K5 1/4W Resistor
• R9-R11______470R 1/4W Resistors
• C1___________47pF 63V Ceramic Capacitor
• C2__________100nF 63V Polyester Capacitor
• C3___________47µF 25V Electrolytic Capacitor
• C4____________1µF 25V Electrolytic Capacitor
• D1______BZX79C5V1 5.1V 500mW Zener Diode
• D2_________1N4148 75V 150mA Diode
• D3-D5________3mm. Yellow LEDs
• IC1_________LM339 Quad Voltage Comparator IC
• SW1__________SPST Slider Switch
• B1_____________9V PP3
• Clip for 9V PP3 Battery

Notes:

• The simplest way to connect this circuit to the amplifier output is to use a twisted pair cable terminated with two insulated crocodile clips.
• Setup is best accomplished with an oscilloscope or an audio millivoltmeter like the one described in these pages. Precision Audio Millivoltmeter
• A 1KHz sine wave generator with variable output is also required (see a suitable circuit in this website also). 1KHz Sinewave Generator
• Connect the generator to the amplifier's input and the Audio Power Indicator to the output of the amplifier, in parallel with the oscilloscope probe or the audio millivoltmeter input.
• When using high power outputs disconnect the loudspeakers to avoid Tweeters damage and connect in their place an 8 Ohm 20-30 Watt wirewound resistor.
• Remember that VRMS output is equal to output Peak-to-Peak Voltage divided by 2.828.
• RMS power output in Watts is equal to VRMS2 divided by speaker impedance (usually 8 or 4 Ohm).
• Example: set the output of the 1KHz sinewave generator to read 14V on the audio millivoltmeter (24.5W @ 8 Ohm). Set R2 until D3 illuminates, and be sure that D3 turns-off when diminishing a little the generator's output.
• Do the same with R7 for D4 and R6 for D5. The readings of the audio millivoltmeter must be 10V (12.5W @ 8 Ohm) and 4V (2W @ 8 Ohm) respectively.

Six-LED Bar Power Indicator

Useful to monitor audio power delivered to loudspeakers
No power supply - no setup required

This device, connected to the loudspeaker output of an audio amplifier, will indicate the instantaneous output power delivered to the loudspeaker(s) by means of six LEDs illuminating one after another by voltage values increasing little by little, providing the visual impression of a luminous bar or column, increasing and decreasing in height following the increase and decrease of the signal's level.


The input signal is first rectified by D1 and then sent to six different voltage dividers, one for each LED. In this way, the indication provided by the LEDs illumination of this "Power Display", will be related to the instantaneous power sunk by the whole loudspeaker cabinet. Six output power levels are displayed by the LEDs in a 2W - 80W range (no setup required). Each nominal power level indication into 8 Ohms load is reached when the respective LED illuminates at full brightness.

Parts:

• R1_____________220R 1/2W Resistor
• R2,R5,R6,R8____100R 1/4W Resistors
• R10,R12,R14____100R 1/4W Resistors
• R3_____________220R 1/4W Resistor
• R4,R7__________330R 1/2W Resistors
• R9_____________560R 1/2W Resistor
• R11____________820R 1/2W Resistor
• R13______________1K2 1/2W Resistor
• D1___________1N4004 400V 1A Diode
• D2,D4,D6__BZX79C2V7 2.7V 500mW Zener Diodes
• D3,D5,D7,D8,D9,D10 Red LEDs (Any dimension and shape) (See Notes)

Notes:

• The output power indicated by each LED must be doubled when 4 Ohms loads are driven.
• The circuit can be adapted to suit less powerful amplifiers by reducing the number of LEDs and related voltage dividers.
• LEDs of any dimension can be used, but rectangular shaped devices will be more suitable to be compacted in bars or columns.
• For a stereo amplifier, two identical circuits are required.

Phone Busy Indicator

Have you ever been using the modem or fax and someone else picks up the phone, breaking the connection? Well, this simple circuit should put an end to that. It signals that the phone is in use by lighting a red LED. When the phone is not in use, a green LED is lit. It needs no external power and can be connected anywhere on the phone line, even mounted inside the phone.

Partlists

• R1 1 3.3K 1/4 W Resistor
• R2 1 33K 1/4 W Resistor
• R3 1 56K 1/4 W Resistor
• R4 1 22K 1/4 W Resistor
• R5 1 4.7K 1/4 W Resistor
• Q1, Q2 2 2N3392 NPN Transistor
• BR1 1 1.5 Amp 250 PIV Bridge Rectifier
• LED1 1 Red LED
• LED2 1 Green LED
• MISC 1 Wire, Case, Phone Cord

Notes:

1. This is a very simple circuit and is easily made on a perf board and mounted inside the phone.
2. LED1 and LED2 flash on and off while the phone is ringing.
3. Do not worry about mixing up the Tip and Ring connections.
4. The ring voltage on a phone line is anywhere from 90 to 130 volts. Make sure no one calls while you are making the line connections or you'll know it. :-)
5. In some countries or states you will have to ask the phone company before you connect this to the line. It might even require an inspection.
6. If the circuit causes distortion on the phone line, connect a 680 ohm resistor in between one of the incoming line wires and the bridge rectifier.

Cut Phone Line Detector

A while ago I got an email asking for the schematic of a circuit to detect cut phone lines. It didn't take me long to find this circuit in Electronics Now. When the circuit detects that a phone line has been cut, it activates a MOSFET which can be used to drive a relay, motor, etc. It can also be connected to a security system.

• R1, R2, R3 3 22 Meg 1/4 W Resistor
• R4 1 2.2 Meg 1/4 W Resistor
• C1 1 0.47uF 250V Mylar Capicitor
• Q1 1 2N3904 Transistor or 2N2222
• Q2 1 2N3906 Transistor
• Q3 1 IRF510 Power MOSFET
• D1 1 1N914 Diode
• Load 1 See "Notes"
• MISC 1 Wire, Phone Connectors, Circiut Board

Notes:

1. The "Load" can be a relay, lamp, motor, etc. The circuit can also be connected to a security system to sound an alarm in case the phone line is cut.
2. If the circuit is connected to a security system or other circuit, both circuits must be electrically isolated from each other using an opto-isolator, relay, etc. This also means that the Cut Phone Line Detector must be powered by a seperate 9V supply.

Fluid Level Detector

The LM903 uses the thermal-resistive probe technique to measure the level of nonflammable fluids. A low fluid level is indicated by a warning lamp operating in continuous or flashing mode. All supervisory requirements to control the thermal - resistive probe, including short and open circuit probe detection, are incorporated within the device. The circuit has possible applications in the detection of hydraulic fluid, oil level, etc., and may be used with partially conducting fluids.


Circuit Operation

A measurement is initiated when the supply is applied, provided the control input pin 7 is low. Once a measurement is commenced, pin 7 is latched low and the ramp capacitor on pin 12 begins to charge. After 25 ms when switch-on transients have subsided, a constant current is applied to the thermo-resistive probe. The value of probe current, which is supplied by an external PNP transistor, is set by an external resistor across an internally generated 21V reference. The lamp current is applied at the start of probe current.

35 ms after switch-on, the voltage across the probe is sampled and held on external capacitor C1 (leakage current at pin 1 less than 1 nA). After a further 1.5 seconds the difference between the present probe voltage and the initial probe voltage is measured, multiplied by 3 and compared with a reference voltage of 850 mV (externally adjustable via pin 16). If the amplified voltage difference is less than the reference voltage the lamp is switched off, otherwise the lamp commences flashing at 1 Hz to 2 Hz. 10 ms later the measurement latch operates to store the result and after a further 8 ms the probe current is switched off.

A second measurement can only be initiated by interrupting the supply. An external CR can be arranged on pin 7 to prevent a second measurement attempt for 1 minute. The measurement condition stored in the latch will control the lamp.

PROBES

The circuit effectively measures the thermal resistance of the probe. This varies depending on the surrounding medium (Figure 1). It is necessary to be able to heat the probe with the current applied and, for there to be sufficient change in resistance with the temperature change, to provide the voltage to be measured.


Probes require resistance wire with a high resistivity and temperature coefficient. Nickel cobalt alloy resistance wires are available with resistivity of 50 mXcm and temperature coefficient of 3300 ppm which can be made into suitable probes. Wires used in probes for use in liquids must be designed to drain freely to avoid clogging. A possible arrangement is shown in Figure 2.

The probe voltage has to be greater than 0.7V to prevent short circuit probe detection less than 5V to avoid open circuit detection. With a 200 mA probe current this gives a probe resistance range of 4X to 25X. This low value makes it possible to use the probe in partially conducting fluids.

Using resistance wire of 50 mXcm resistivity, 8 cm of 0.08 mm (40 AWG) give approximately 8X at 25ßC. Such a probe will give about 500 mV change between first and second measurements in air, and 100 mV change with oil, hydraulic fluid, etc., in the application circuit. With an alarm threshold of 280 mV (typ) lack of fluid can readily be detected. As the probe current, measurement reference and measurement period are all externally adjustable, there is freedom to use different probes and fluids.

Another possibility is the use of high temperature coefficient resistors made for special applications and positive temperature coefficient thermistors. The encapsulation must have a sufficiently low thermal resistance so as not to mask the change due to the different surrounding mediums, and the thermal time constant must be quick enough to enable the temperature change to take place between the two measurements. The ramp timing could be adjusted to assist this. Probes in liquids must be able to drain freely.