The counter

The circuit of this device is quite simple: an input circuit filters the signal provided by the antenna to reduce sensibility to other phenomena and protects against over voltages. The four transistors realize a "one-shot" section that is triggered by the input circuit and provides pulses of about 0.05 seconds to the counter (the 470 nF capacitor set this time). When the one-shot is triggered it cannot be retriggered for about one second (imposed by the 8 uF polarized capacitor) to avoid multiple counts for the same strike.
This part is powered by a 6LR61 9 V cell and, since the power consummation is lower than 1 uA (when no strikes are detected) a single cell will last for some years. The test button allows to check if the battery is still operational: each time it's pressed the counter should increment and, if hold down, the counter should increment about once per second.
The two optocouplers interface the output of the one-shot circuit with the real counter and with the PC. I used a digital 5-digits LCD counter powered by a LR6 (AA) 1.5 V cell. The power consummation of this counter is about 1.5 uA and the battery life is about one year. This counter also has a reset switch and a piezo speaker that beeps on every operation. Any other counter should work fine (even mechanical ones) but the interface must be modified. The insulation between the counter and the one-shot provided by the optocoupler here is not necessary: I just used an optocoupler to adapt the two different voltages (9 V and 1.5 V). A 6-9 V DC counter can be directly connected between the collector of the 2N2904 ant the ground.
The other optocoupler interfaces this circuit with a PC via an RS232 serial port. Here the insulation is necessary.

(Click to enlarge)

This circuit is not critical and every semiconductor can be replaced with a similar one. The input circuit uses power resistors and high voltage capacitors: it must be well insulated. For better accuracy the input circuit should use 1% precision parts.

The whole circuit must be mounted in a grounded metallic box.

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