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Simple Alarms
In the circuits shown below, the diode should be permanently illuminated by a beam of light.  
When the light beam is cut, by an intruder, the conductivity of the diode decreases and the alarm sounds.
The alarm will continue to sound for a long time, even when the light returns, until the reset switch is pressed.  
The value of resistor R is found by experiment but will probably be between 100k and 560k (start with 180k.)  
The i.c. is a 4093 quad NAND Schmitt trigger.  
   
If you want an alarm which sounds when the diode is exposed to light (for example, in order to protect a bag, drawer, suitcase etc) you can interchange the positions of the diode and resistor R.  
   
The first NAND gate is used simply as an inverting switch.  
The second gate is used to make an oscillator which can drive a high impedance earphone or small speaker.  
The components Rí and C determine the frequency of the oscillator and therefore the pitch of the sound produced by the alarm; try 560k and 1.5n.  
   
 
   
Vero board, copper side.  
 
   
Vero board, component side.  
 
   
If the other two Schmitt triggers in the 4093 are used as shown below, you can have an alarm which gives a series of "beeps" rather than a continuous tone.  
   
 
   
Vero board, copper side.  
 
   
Vero board, component side.  
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