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Optically Isolated Triac Driver
A triac can be used to make something similar to a relay but without any moving parts.  
If you want to make, for example, the sound operated switch or person counter, operate a 220V light this circuit could be used.  
   
If a triac is to be used to switch currents at 220V, the high voltage part of the circuit must be well insulated from the low voltage parts.  
If this is not done and the triac develops a fault, it might feed the 220V into the low voltage part of the circuit.  
This would, of course, be disastrous for the low voltage part of the circuit but it could also be very dangerous for the user.  
For this reason:  
TAKE EXTRA CARE WHEN SETTING UP CIRCUITS LIKE THIS ONE!  
   
One simple way of isolating the two parts of the circuit is shown below.  
I used a high intensity red led but I assume a modern white led would be equally effective.  
Using the red led, the triac can be switched using a current as low as ½mA.  
This means that the circuit can be connected directly to the output of any op. amp. or CMOS logic chip.
Of course, special opto-isolator chips can be bought but it's amusing to make your own.  
   
 
   
The resistor in series with the led should be 3k9 for use with a 5V circuit and 6k8 for 9V.  
   
This circuit has been tested and found to work well using a large (10mm diameter) l.d.r. and the following triacs: BT137F (600v), BTB 12 700.  
It can probably be made to work with most high power triacs but is NOT suitable for use with triacs having a very sensitive gate (for example TIC225, TIC226, ...).  
   
For a method of making simple printed circuit boards, like this one, see here.  
   
Printed Circuit Board, Copper Side  
 
   
Printed Circuit Board, Component Side  
 
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