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Circuit Operation - Readout Unit
Note that the component reference numbers start at 1 on both the Sender and Readout. Try not to get confused!

IC1b and IC1c form an oscillator, the same as the one in the sender unit. One resistor value is variable over a range of +/- 10%, so that the unit may be calibrated. The output is connected to the Clock input of IC2

The frequency of this type of oscillator varies with different makes of 4093 IC. To avoid any problems use the same make of 4093 IC in the Sender and Readout units. This can usually be assured by purchasing them from the same place at the same time.

IC2 and IC3 are decimal counters with decoded 7 segment outputs. These outputs are connected to the displays via emitter follower circuits. The carry output (CO) of IC2 is connected to the clock input of IC3, so IC3 is incremented when IC2 steps from nine to zero. IC3 thus drives the tens display while IC2 counts the units.

The test probes are connected to SK1 ("Non-Reading") and SK2 ("Reading"). The "Reading" input is protected by R1, D1 and D2, as described earlier. R2 holds the input low when it is not connected, blanking the display to conserve battery power. IC1a inverts the signal. When the unit is connected to the Sender, the output of IC1a is normally low, and goes high on the Sender output pulse.

When the input is high (IC1a output low), the clock inhibit inputs (pin 2) of IC2 and IC3 are held high (via IC1d), so that the counters do not respond to the clock input. The display enable inputs (pin 3) are also held high, so the displays are illuminated.

When the input goes low, the reset inputs (pin 15) of IC2 and IC3 are pulsed high momentarily high, reseting the counters. This brief reset pulse is also coupled to the oscillator via D7, to bring the oscillator into line with the Sender unit. The clock inhibit input goes low, allowing the counters to count the clock pulses and the display enable inputs also go low, blanking the displays. This blanking period is brief, and hardly visible in practice.

When the input goes high again, the displays show the number the counters reached. This number depends on the length of time the input was low, and is therefore the wire number.

The power supply arrangement is identical to that in the Sender unit.

PCB Construction
Because both PCB's use the same component references, the parts list has been divided into two distinct sections. To save confusion, it may be easier to completely build one PCB at a time.

The PCB construction is very straightforward, and requires little comment from me. Note that some wire links pass under components, so these should be fitted first. Use terminal pins or single-in-line header strip for the off-board components - particularly on the Sender PCB - these will make wiring up much easier. Drill a hole in the Readout PCB below VR1 to allow it to be adjusted once it is fitted into the case.

Do not fit the LED's yet. In the Sender unit the LED is mounted on the case, so terminal pins should be fitted in the PCB. In the Readout the LED has to fit through a hole in the case so it would be better to solder it once everything has been lined up.

On the prototype, the IC's and LED displays were soldered directly into the PCB. You may find it useful to use sockets for the displays to space them away from the PCB.

You may also wish to fit the IC's in sockets, particularly IC6 through IC13 on the Sender unit, since these are most likely to be damaged if the unit is inadvertently connected to low voltage live wires. The same applies to IC1 on the Readout unit. If the units are accidentally connected to live mains voltage wiring, they will almost certainly be damaged beyond repair.