Fastest Finger First project

Posted on Aug 24, 2012

If 12V relays are used, you can replace the 180R resistors R7,R9,R14,R16,R20,R22,R26 and R28 with a wire link (the resistors are only there to limit current if you use 5V relays, of which I had a load in my parts bin). If the system doesn't reset cleanly on power-up (several lights may come on or the relays may chatter), just solder a 100nF capacitor across the RESET button. This will cause a reset pulse to be generated at power-up so that the initial state is defined. If you don't want the nice `beep`, leave out the NE555 (IC6) and its associated components. Note that the buzzer needs to be an `active` one, i.e. one that includes its own drive electronics such that simply applying a DC voltage makes it sound. If you want to use less or more button/light positions just omit/include some of the replicated logic (it's fairly clear from the schematic above). Note the pin/contact layout of the relays used. Not all DPDT DIL relays use the same layout - be careful! R32, the 10K trimmer, is used to vary the length of the on time of the buzzer between about 0.25 and 1 second - I found that about 0.5 seconds was quite enough. The prototype was built on VeroBoard (StripBoard in the USA). If you use roughly the same positions for components as shown in the board layout above, you won't go far wrong. How does it work? Each user station has a button connected between Vcc and its BNx line, and a 12V light connected between its LTx line and GND. For each user, a copy of the circuit below is on the main circuit board. This controls handling of the user's button and driving the output relay.

Fastest Finger First project
Click here to download the full size of the above Circuit.

When any user presses their button, the CAP line will go high. As buttons always 'bounce', there will be a whole raft of pulses in the first few milliseconds before the contacts in the button settle down. IC1A is configured as a monostable that drives the CK line - this will go low for about 100mS when the monostable is triggered. Remember that 4013 RS flip-flops are positive edge triggered, so when IC1A times out, the CK line will go high. If any button is still pressed, the BNx line will be latched through to its Q output (see IC2A above), turning on its relay. At the same time, IC1B will latch the CAP line to its Q output. If any button is pressed, this will cause IC1A's RESET input to go high, forcing the CK line low regardless of any other pulses on the CAP line, thus debarring other users who may subsequently press their buttons, i.e. IC1A's monostable cannot be fired again until the RESET button is pressed. This mechanism ignores noise spikes and bounces on any button line, and completely blocks other users once the first user has pressed their button. The only way one or more outputs may be triggered at the same time is if the users press their buttons within IC1A's timeout period (about 100mS), thus indicating a draw. Because of the high noise immunity of the circuit, you can use multi-core cable to connect the control unit to the users' boxes. Each box requires 4 wires - two of these,...

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