Idea: Elex team
Imagine this – you are comfortably in bed, have turned off the light are are waiting the return trip to Slumberland. And then: beep... beep... beep... What miscreant is making that noise? A mouse or a cricket or...? Turn on the light to have a look – but it is suddenly quiet again. Nothing to see, nothing to find, so turn the light off again. And then after some time the wretched beeping starts again...
This is the scenario of our electronic tormentor: an electronic cricket that, thanks to a light-sensitive element, is only active in the dark; once the victim turns on the light it goes completely quiet. The schematic is simplicity itself: see Figure 1.
Light-sensitive resistor R11, together with potentiometer P1, forms a light-dependent voltage divider. When lighted, transistor T1 iconducts and the voltage across capacitor C1 is practically 0 V. The voltage at the output of the second Schmitt-trigger (N3/N4) is then low. The sound generator around IC2 (the ‘good old’ 555) is then inactive through the (active low) reset input, pin 4.
Now the light is switched off and the fun begins. The voltage at the junction of R11 and P1 increases and at a certain point the Schmitt-trigger N1/N2 will toggle. The output drops low and T1 turns off. After some time (depending on the value of C1), the voltage at the junction of R5 and C1 will be high enough to cause Schmitt-trigger N3/N4 to toggle, and the 555 is released from its reset state.
The value of R8 is used to control the volume of the beep (minimum is 100 Ω), and the light sensitivity is adjusted with P1.
Disclaimer: The use of the electronic tormentor is entirely at your own risk. Elektor accepts no liability for black eyes or the like...