Nuts and Volts - July/August 2018

If you want a pulsing sound, then you should change the timing components (R8 = 47K, R9 = 15K, C4 = 4. 7 µf) and use an internally driven buzzer.

You can easily use the PIC 555 circuit of Schematic

2 for this application. Use the circuit shown in

Schematic 9C to drive the MCLR input and use mode

4. You can set the circuit to either have a continuous tone or a pulsing tone just as described for the 555 implementation in the previous paragraph.

Mims Circuits 26 and 27

The two circuits shown in Schematic 10 are an analog lightwave transmitter and receiver. The transmitter circuit is a simple 555 astable multivibrator whose output period varies with the value of R1. If you use a photoresistor for R1, the frequency will vary with the amount of light hitting it. The higher the light intensity, the lower the resistance which yields higher frequencies.

The 555 portion of the receiver is identical with a few minor component value differences to the frequency meter of FM- 13 discussed in Part 2 of this series. The op-amp is there to amplify the signal from the phototransistor so that relatively weak signals can be handled. The op-amp circuit can be eliminated if the transistor is close to the IR diode.

The operation of the circuit is such that the meter — when calibrated with R109 — will show the frequency of the transmitter pulse stream. R104 is used to adjust the trip point of 1458b which is being used as a comparator.

There is one minor issue with the circuit: The 1458 specifications state that the minimum voltage between the power pins is 10V. The circuit will probably work at 9V, but it could be marginal. The text in the book suggests that 741s may be substituted for the 1458; however, the 741 specifies a minimum supply of ±10V or, if used with a single supply, 20V.

Schematic 11 shows an alternate op-amp circuit. The basic circuit is the same except that the op-amp has been replaced with one of the many rail-to-rail op-amps available today which were not available when the Engineer’s Workbook series was written.

This circuit has two additional features for section 2 of the op-amp: no potentiometer required for setting the trip point; and some hysteresis (about 100 mV) for a little noise immunity. The op-amp shown has a maximum supply voltage of 6V. However, there are many single voltage rail-to-rail op-amps that have a wider supply voltage range which will also work in this circuit.

Two PIC replacements can be used instead of the

555s. Use the table shown back in Schematic 2 to select

July/August 2018 63

Resources All schematics are drawn using Dip Trace www.diptrace.com All parts purchased are from Digi-Key www.digikey.com My website www.qsl.net/k3pto