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Just when you get all comfy in bed you hear it…. Drip….Drip….Drip. But when you stumble into the bathroom to give the faucet a twist, the dripping sound stops. After a few trips back and forth you venture out without turning on the lights- to “sneak up” on the drip. The only problem is that you can’t see anything! Well, this fate can befall a friend (hereafter referred to as “victim”) with a little help from you and the Dripper. The Dripper produces a slow dripping sound but only when the lights are off. The circuit is based on a low power version of the two-transistor flasher and should run on a 9 volt battery for weeks.
This new circuit uses regeneration to increase the ringing of the tuned circuit that produces the chirp sound. The circuit is therefore less sensitive to transistor gains and inductor Q. The potentiometer across the inductor controls the chirp length. A short chirp will sound most like a drip whereas a longer chirp sounds like some sort of insect. The CDS photocell is optional and will cause the dripping to stop when the lights are turned on.
Here is how it works:
When dark, the photocell has a very high resistance and the voltage on the base of the 2N4403 is set by the two resistor to about 6 volts. The emitter capacitor begins charging toward this value through the 330k resistor which determines the drip rate. When the emitter voltage rises above the base voltage the two transistors turn on and the 2N4401 quickly begins to oscillate at an audio frequency. The 22uF capacitor on the emitter quickly discharges and the circuit switches off and the whole process begins again. The turn on click followed by the short audio oscillation creates a realistic “pit – tink” dripping sound when the pot is adjusted properly. Different values will give different effects and experimentation can be entertaining. Transistor substitutions will usually work well.