Pad Type Device
Construction of this device is rather straightforward. Parts: B1-B4 any good 9 Volt Alkaline battery, When changing batteries you must change all four with the same type. Battery life is between 100 and 200 hours of use. J1 is a BNC connector that is used with a BNC jumper to go to your function generator. You can omit the connector and solder the shielded cable to the circuit board. S1 DPST or DPDT power switch. R1 4.7K 1/4 Watt 5% resistor. R2 200K 1/4 Watt 5% resistor. R3 150K 1/4 Watt 5% resistor. R4 100K Linear Taper Potentiometer 1/10 Watt. R5 34K 1/4 Watt 5% resistor. J2 & J3 Banana Jack. U1 TS522 Duel Op-Amp this part can be substituted with TL082, MC1458, MC33078, LM358, etc. TS522 was used because of the high frequency cutoff point well over 1 Mhz. as well as other factors. If you want to use this circuit in the upper frequency range then a ground plane on the circuit board as well as capacitors on the power rails is suggested. To use this circuit make sure that your function generator is a square wave 50% Duty cycle and that the output is TTL not CMOS. Op Amps are set to trip at 2.5 Volts with the R2 and R3 voltage divider. Output current is limited to a maximun of 1 Ma by R5 which can be changed to a lower value.
You can run this device with a free Computer Function Generator. You will need to download the program and run it on a machine with windows 3.1 or 95/98. You will also need a 1/8" Stereo plug and cable to plug into your sound card. Only one channel will be used in this setup. Make sure that the function generator is on Square wave and the volume is turned up. When used on a sound card R3 may need to be changed to 160K or 180K. This will change the trip voltage to 2 Volts for 160K and 1 Volt for 180K.
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