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
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
All schematics are drawn using Dip Trace
All parts purchased are
SCHEMATIC 10. 555 analog lightwave transmitter and
SCHEMATIC 11. Alternate op-amp buffer.