![]() ![]() The 39k resistor on the base of the 2N4401 sets the base current to (1.5V-0.7V)/39k = 20 micro amps. When the signal swings to ground, the 2N4401 in the alarm module sinks enough current to pull the interrupt pin low. The alarm signal is taken from one of the wires from the alarm clock module to the clock’s speaker. To raise the voltage to 2.7V or greater to run the AT90S2313, I added a second AA cell that only powers the AT90S2313. The clock module runs from a single 1.5 volt AA cell. The speaker outputs are configured as outputs (I hope that is not a surprise). The digital inputs are pulled high with the chips’ weak internal pull-ups to minimize power dissipation. Once the “WAKE UP” message has been sent, the micro controller goes back to sleep. The 100 uf capacitor in series with the speaker keeps it from drawing any DC power. Interrupts provide a 2 kHz square wave on pins 14 and 15 that are 180 degrees out of phase with one another (when one pin is high, the other pin is low), which means that the speaker sees a square wave that is nearly 6 volts peak-to-peak (or 3 volts RMS since it is a square wave). ![]() ![]() and wakes up, then, using timing loops, it generates timing for “WAKE UP” in Morse code. The micro controller sleeps most of the time, consuming mere micro amps until it receives an interrupt. ![]()
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