A simple two tone water alarm that is light enough to be worn on the upper arm of a sleeper is described. It uses two LMC555 CMOS timer chips followed by a complimentary pair of emitter followers to drive an 8 ohm speaker. Power is supplied by a 9 volt transistor radio battery. An modification to produce a siren rather than two tone operation is discussed. A little experimentation with a signal generator showed that it would be very loud when driven with only 3 or 4 volts between 800 Hz and 3 kHz. I thought about ways to drive the speaker: use an LM324 quad op-amp followed by a pair of transistors, make a transistor multivibrator and have that drive a transistor buffer, make a couple of oscillators with a hex inverter. Just about the, the UPS delivery truck pulled up and I took delivery of a package from Mouser Electronics, which included among its contents, some LMC555N timer chips that I had ordered in anticipation of a very different project.
This was the solution. The LMC555's can be set up to oscillate with a 50% duty cycle with only a single resistor and capacitor -a improvement over the recommended circuit for the old bipolar NE555, it can operate directly from a 9 volt transistor radio battery, and can be frequency modulated by injecting a signal into the control input (pin 5). The circuit shown in Figure 1 is the one that that fell into place. According to the data sheet, U2's oscillation frequency, F can be found by:
F = 1/(1.4 R C)
I wanted to run around 2.5 kHz, and chose a 100k resistor, and found the ideal value of the capacitor (from the formula above):
C = 1/(1.4 R F) = 1/(1.4 x 100K 2500 Hz) = .0028 uf. So, I used the closest standard value I had on had, which is .0027 uf.
The output of the oscillator drives the complimentary pair made of the 2N2222 and 2N2907. The complimentary pair is necessary because the peak current into the speaker can reach several hundred milliamps, and the LMC555 cannot drive it directly. This pair has a tremendous amount of crossover distortion, so it would not be useful as shown here for normal audio use, but in this circuit, it is being driven by a square wave, and that distortion should not have any effect on the signal, other than to reduce the amplitude a little bit.
A volume control was inserted between the output of the LMC555N (pin 3) and the bases of the output buffer transistors, because the speaker...