The "user interface" to the "trap chess" board is (beyond a "start" switch) the classic chess dual-timer that each player taps to record the elapsed decision time.

Bob got a cheap but nice looking timer that I reverse-engineered to get to the useful signals, mainly the top switch, which triggers my machinery when it changes state.

First, the timer operates on a single 1.5V AA battery. There is no point in having it installed inside the timer so I power the timer with a ... "shunt regulator". 1.5V is more or less the drop of 2 silicon diodes so here we go : 2× 1N4148 in series, driven by the main 5V supply. Now, usually, you use a resistor to limit the current, but I also used an old red LED in series, to show that the circuit is powered.

The timer draws very very little current but the diodes need to pass quite a lot of current in order to get a suitable drop. The resistor in series with the LED sets the current at around 10mA, and most of it is lost.

The LED also acts as a reverse-blocking diode for when the voltage drops (due to electromechanical activity). A capacitor (100µF) keeps some charge during the transients. Actually, the diodes' leakage and the timer's consumption are so low that the timer won't reset by itself... I need to really remove the connection with 1.5V for the timer to restart correctly later.

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The top switch (lap switch ?) has no physical contact (to avoid wear). A reed magnetic switch does the work though. Now, the switch is not powered on permenently : it's polled during 2ms every 32ms. I could patch a wire that sends a train of pulses when the switch is closed, and no pulses when open.

I reused the previous circuit, using 2 × BJT, to filter that low-level signal, but development was not as easy as expected. Using my scopes, I could trace and understand my various mistakes and false assumptions. Now a LED is on when the top switch is in one position :-)

(scanned schematics to follow soon)