GPIB to RS-232 converter

This project fills the need of anybody who has a test instrument with the GPIB port and likes to get the screen dump on his PC without any GPIB card. It emulates the HP7470A operation on the GPIB side, and outputs the HP-GL data at the RS-232 port to be read and stored on the PC by any suitable software. The operation of this interface is not just limited to plotter emulation: any data intended to be received by a GPIB Device (addressable or listener only) can be captured and brought out to the RS-232 port, including raw data from the instrument or rasterized data for a GPIB graphic printer. GPIB addresses and other set-and-forget parameters can be modified and permanently stored using a simple setup menu. It is based on a PIC16F628A microcontroller, and the PCB size is just 7x7.5cm.



SCHEMATIC AND DETAILS

The hardware of Pic-Plot interface is quite simple: the active components are just a PIC16F628A, a 5V regulator and three transistors. External connections are a GPIB connector, a Serial port and a DC power connector.



The microcontroller does all the necessary jobs to emulate GPIB Device functionality, in both Listener and Talker mode, by recognizing addressing, commands and managing the Handshake lines. Controller mode is not needed for the intended functionality, and therefore is not supported. Once the device is addressed and it receives data from the Talker, the same data are forwarded to the COM port at 9600 baud: the hardware UART inside the PIC16F628A generates the serial data going to the PC through the RS232 port. Only in Setup mode the data flow is bidirectional at the same baud rate. The RS-232 connector on the interface is a standard male DB-9, and should be connected to the PC using a null-modem female-to-female serial cable.

PCs missing the COM port but equipped with an USB port can be still used with the aid of an inexpensive USB to serial converter, provided that the necessary Virtual Com Port drivers are properly installed.

A jumper is provided to enter Setup mode: when Pic-Plot it powered with this jumper in the closed (short) position, then it starts-up in Setup mode. In this mode the microcontroller UART is used to read/change a few set-and-forget parameters. GPIB cable can be left connected to the instrument, but in Setup mode the GPIB port is not monitored by the Pic-Plot. For normal operation this jumper must be left open. More details about Setup mode can be found in the USE AND OPERATION section.

Power supply can be any voltage between 8 and 16V, and current drain is far below 20mA. With such a large supply requirements, a low-cost unregulated 12V wall adaptor can be used as a power source. Connector polarity is center positive (+). An interesting possibility for those who use the USB-to-serial bridge is to bypass the Pic-Plot 5V onboard regulator and spill the 5V supply from the USB connector mounted on the bridge. This solution of course asks for a simple modification of the bridge or a modified USB cable, then it is suggested only to people having the necessary technical skills to do things right. You can find details by clicking here, or you might prefer to see our new Pic-plot2 which directly supports GPIB-USB conversion.

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.

Stepper Motor Controller

The circuit is very simple and inexpensive. This is good thing because most commercial stepper motor controller ICs are quite expensive. This circuit is built from standard components and can easily be adapted to be controlled by a computer. If you use cheap surplus transistors and stepper motor, the price of the circuit can be kept to under $10.


Parts

• R1, R2 ,R3, R4 ------ 4 1K 1/4W Resistor
• D1, D2, D3, D4 ------ 4 1N4002 Silicon Diode
• Q1, Q2, Q3, Q4 ------ 4 TIP31 NPN Transistor (See Notes) TIP41, 2N3055
• U1 ----------------- 1 4070 CMOS XOR Integrated Circuit
• U2 ----------------- 1 4027 CMOS Flip-Flop
• S1 ----------------- 1 SPDT Switch
• MISC ----------------- 1 Case, Board, Wire, Stepper Motor

Notes

• You should be able to substitute any standard (2N3055, etc.) power transistor for Q1-Q4.
• Every time the STEP line is pulsed, the motor moves one step.
• S1 changes the motors direction.

AC Motor Speed Controller

This AC motor speed controller can handle most universal type (brushed) AC motors and other loads up to about 250W. It works in much the same was a light dimmer circuit; by chopping part of the AC waveform off to effectively control voltage. Because of this functionality, the circuit will work for a wide variety of loads including incandescent light bulbs, heating elements, brushed AC motors and some transformers. The circuit tries to maintain a constant motor speed regardless of load so it is also ideal for power tools. Note that the circuit can only control brushed AC motors. Inductive motors require a variable frequency control.


Parts

• R1 ------- 1 27K 1W Resistor
• R2 ------- 1 10K 1/4W Resistor
• R3 ------- 1 100K 1/4W Resistor
• R4 ------- 1 33K 1/4W Resistor
• R5 ------- 1 2.2K 1/4W Resistor
• R6 ------- 1 1K 1/4W Resistor
• R7 ------- 1 60K Ohm 1/4W Resistor
• R8 ------- 1 3K Linear Taper Trim Pot
• R9 ------- 1 5K Linear Taper Pot
• R10 ------- 1 4.7K Linear Taper Trim Pot
• R11 ------- 1 3.3K 1/4W Resistor
• R12 ------- 1 100 Ohm 1/4W Resistor
• R13 ------- 1 47 Ohm 1W Resistor (See Notes)
• C1, C3 ---- 2 0.1uF Ceramic Disc Capacitor
• C2 ------- 1 100uF 50V Electrolytic Capacitor
• D1 ------- 1 6V Zener Diode
• Q1 ------- 1 2N2222 NPN Transistor 2N3904
• SCR1 ----- 1 ECG5400
• TR1 ----- 1 TRIAC (See Notes)
• U1 -------- 1 DIAC Opto-Isolator (See Notes)
• BR1, BR2 -- 2 5A 50V Bridge Rectifier
• T1 ------- 1 Transformer (See Notes)
• MISC ----- 1 PC Board, Case, Line Cord, Socket For U1, Heatsinks

Notes

• TR1 must be chosen to match the requirements of the load. Most generic TRIACs with ratings to support your load will work fine in this circuit. If you find a TRIAC that works well, feel free to leave a comment.
• U1 must be chosen to match the ratings of TR1. Most generic DIAC based opto-isolators will work fine. If you have success with a specific part, feel free to leave a comment.
• T1 is any small transformer with a 1:10 turns ratio. The circuit is designed to run on 120V so a 120V to 12V transformer will work. Alternately, you can wind T1 on a transformer core using a primary of 25 turns, a secondary of 200 turns, and 26 gauge magnet wire.
• R9 is used to adjust motor speed. R10 is a trim pot used to fine tune the governing action of the circuit. R8 fine tunes the feedback circuit to adjust for proper voltage at the gate of SCR1. It should be adjusted to just past the minimum point at which the circuit begins to operate.
• R13 must be chosen to match the load. Generally, larger loads will require a smaller value.
• Since this circuit is not isolated from mains, it must be built in an insulated case.

Computer Controlled Frequency Counter/Logic Probe

This circuit is a stable frequency counter accurate to 5 significant digits. The range is 0 - 30MHz with an input sensitivity of greater then 100mV. The probe connects to the PC serial port. So by using the crystal oscillator already present on your PC serial card and software calibration, the Probes' external circuitry is kept to a minimum. Probe 9 can also be used as a logic probe/analyzer using included software (LPROBE92.EXE).


Parts

• R1,R2,R3,R4---- 4 100K 1/4W Resistor
• R5 ------------ 1 10M 1/4W Resistor
• R6, R7 ------- 2 3.3K 1/4W Resistor
• R8 ----------- 1 390 Ohm 1/4W Resistor
• R9 ----------- 1 1M 1/4W Resistor
• C1, C4 ------- 2 0.1uF Ceramic Disc Capacitor
• C2, C3 ------- 2 100uF 16V Electrolytic Capacitor
• D1 ----------- 1 1N4148 Signal Diode Any 200mA silicon signal diode
• D2, D3 ------- 2 3.3V Zener Diode
• D4 ----------- 1 6.2V Zener Diode
• U1 ----------- 1 74HC00 Quad Highspeed NAND Gate
• U2, U3, U4 ---- 3 4021 8 Stage Shift Register
• U5 ----------- 1 74HC393 Dual Highspeed 4 Bit Counter
• U6 ----------- 1 4040 12 Stage Binary Counter
• MISC --------- 1 PC Board, Wire, Suitable Probe, DB9/DB25 Connector

• The software to use this probe can be downloaded using the following link. Note that this software is compiled for Intel x86 platforms and runs under DOS, Win95, Win98 and WinMe. It does not run under any Windows version based on NT including Windows NT 3.51, WinNT4, Win2K, Win2K3, WinXP and Windows Vista. This is because NT based operating systems do not allow direct hardware access.

Probe Software, Zipped, 19K

• SETPROBE.EXE is the frequency counter calibration program. To give accurate readings the Probe must be calibrated to your PC. SETPROBE.EXE calculates the constant error correction factor for the particular PC serial card the probe is to be used on. The frequency counter corrects for this slight constant error in crystal frequency by using the correction factor contained in PROBE.DAT. To calculate this correction factor, a reliable oscillator of known frequency (eg 2MHz Crystal Oscillator) is required. When CALIBRAT.EXE is run, the Probe will sample the frequency and then ask for the true frequency value in HZ. The frequency entered must be to 1 Hz accuracy (no decimal points) or an error will occur (for example "200123" not "200123.34" or "2003.421 kHz"). The program then calculates constant error correction factor and stores it to PROBE.DAT. Calibration is only necessary once.
• LPROBE92.EXE is the logic analysis program . Logic states are displayed in real time. This program runs best under DOS (not a DOS window). The sampling speed is adjusted by using the left and right arrow keys.

The three triggering modes are:

• TRIG: Starts each scan (left-right of screen) on a negative going edge of logic signal.
• KEY TRIG: Waits for a key to be pressed before beginning each scan.
• FREE RUNNING: Not triggered.

• To Toggle between these use the UP / DOWN arrow keys. To quit from LPROBE press escape.
• FPROBE92.EXE is the frequency counter program. The measured frequency is displayed in Hz with commas indicating KHz and MHz. To quit from FPROBE press any key.
• Serial port pinouts are as follows.

Automatic Loudness Control

Simple add-on module
Switchable "Control-flat" option

In order to obtain a good audio reproduction at different listening levels, a different tone-controls setting should be necessary to suit the well known behaviour of the human ear. In fact, the human ear sensitivity varies in a non-linear manner through the entire audible frequency band, as shown by Fletcher-Munson curves.

A simple approach to this problem can be done inserting a circuit in the preamplifier stage, capable of varying automatically the frequency response of the entire audio chain in respect to the position of the control knob, in order to keep ideal listening conditions under different listening levels.

Fortunately, the human ear is not too critical, so a rather simple circuit can provide a satisfactory performance through a 40dB range. The circuit is shown with SW1 in the "Control-flat" position, i.e. without the Automatic Loudness Control. In this position the circuit acts as a linear preamplifier stage, with the voltage gain set by means of Trimmer R7. Switching SW1 in the opposite position the circuit becomes an Automatic Loudness Control and its frequency response varies in respect to the position of the control knob by the amount shown in the table below.
C1 boosts the low frequencies and C4 boosts the higher ones. Maximum boost at low frequencies is limited by R2; R5 do the same at high frequencies.

Parts:

• P1_________________10K Linear Potentiometer (Dual-gang for stereo)
• R1,R6,R8__________100K 1/4W Resistors
• R2_________________27K 1/4W Resistor
• R3,R5_______________1K 1/4W Resistors
• R4__________________1M 1/4W Resistor
• R7_________________20K 1/2W Trimmer Cermet
• C1________________100nF 63V Polyester Capacitor
• C2_________________47nF 63V Polyester Capacitor
• C3________________470nF 63V Polyester Capacitor
• C4_________________15nF 63V Polyester Capacitor
• C5,C9_______________1µF 63V Electrolytic or Polyester Capacitors
• C6,C8______________47µF 63V Electrolytic Capacitors
• C7________________100pF 63V Ceramic Capacitor
• IC1_______________TL072 Dual BIFET Op-Amp
• SW1________________DPDT Switch (four poles for stereo)

Technical data:

Frequency response referred to 1KHz and different control knob positions:
Knob position table

Total harmonic distortion at all frequencies and 1V RMS output: <0.01% style="font-weight: bold;">

Notes:

• SW1 is shown in "Control flat" position.
• Schematic shows left channel only, therefore for stereo operation all parts must be doubled except IC1, C6 and C8.
• Numbers in parentheses show IC1 right channel pin connections.
• R7 should be set to obtain maximum undistorted output power from the amplifier with a standard music programme source and P1 rotated fully clockwise.

VO Fuel Controller

A small circuit to avoid cross contamination of diesel and VO in dual tank vehicles. This is a schematic for a vegetable oil fuel controller, the function is to enforce that VO goes back to the VO tank and diesel goes back to the diesel tank.

Notable Parts:

1. K1 is the return fuel line relay
2. K2 is the send fuel line relay
3. S1 is the primary switch
4. S2 is the bypass switch
5. S3 is the purge button
6. S4 is the On/Off switch
7. LED1 is the return line indicator
8. LED2 is the send line indicator (download schematic)

Usage

When the vehicle is warm enough so that it can run on vegetable oil turn on S1. The send line will immediately switch to VO and the return line will stay on diesel for a user specified amount of time. To determine correct timing switch your engine to VO and time how long it takes for the diesel to be purge from the system. Now you set the time in the circuit by changing R1 to the correct value based on 1.1 * R1 * C2. To make it easier, I set C2 at 1000µF, so if you want about 45 seconds use the closest value below 45Kohms (45,000 ohms). In the circuit as set up above R1 is 39K ohms giving a timing of just under 45 seconds (1.1 * 39 = 42.9).

When you are a few minutes from home turn off S1 and press S3. By turning off S1 you will switch the send line back to diesel and by pressing S3 you will keep the return line on VO for a user specified amount of time. To set timing use the same value resistor for R4 as you did for R1.

If you stop for a short period of time and the engine is still warm enough to run on VO when you restart it then either switch on S2 for a minute or so or press the purge button. In either case you will bypass the on-delay timer and keep the VO going to the VO tank.

Caveats

Do not expect exact timing from this circuit because capacitors are not perfect and voltage leakage will increase the time to some extent. When I timed the above circuit I found that it varied approximately 2 - 5 seconds (though I used a stopwatch and might have hit the start early or late, so YMMV). The timing can also be affected by length of time of discharge of the capacitors. If you turn off the circuit and turn it on again pretty quickly the timing can be much shorter than expected. I do not consider this an issue because the time it takes for the vehicle to cool down should be well longer than the time it takes for the capacitors to discharge. If this does become a problem use a lower value capacitor and a higher value resistor, for instance you can use a 220uF capacitor and a 180K ohm resistor to get approximately the same amount of time but the timing errors I initially stated may become more noticeable.

Modifications

If you want the circuit to be more automated so you can just switch it on when you turn on the vehicle and it will wait until temperatures are high enough before switching from diesel to VO, just add a thermostat into the circuit directly before S1. Use a NO (normally open) thermostat set to close it's contacts when the desired temperature is reached.

Parts List

1. (1) 7805 voltage regulator
2. (2) 1N4148 diodes
3. (2) SPST switch
4. (1) DPST switch
5. (1) N/O momentary push button switch
6. (2) LM555 timer
7. (2) 1000uF polarized capacitors
8. (1) 0.01uF non-polarized capacitor
9. (3) 0.1uF non-polarized capacitors
10. (2) LEDs
11. (2) 500 ohm resistors
12. (2) 100K ohm resistors
13. (2) resistors chosen for timing value (R1 and R4)
14. (2) solid state relays capable of handling the current your solenoid valves draw

All capacitors should be rated at least 25 volts, anything higher is fine.
Resistors should be rated for 1/4 watt.
7805 is a generic voltage regulator, if it says 78L05AZ or something it's still fine. visit page