Hi everyone,

I'm building a theft alarm system for my car and for the final alarm section, I need a delay ckt for 5 seconds. It is intended to run a warning buzzer to notify if I accidently left it on before the main alarm goes on.. Is it possible with a normal version of 555?? I dont want the CMOS type cos of some availabilty issues.. If 555 cant do, a transistor with capacitor would be enough just for the delay. how can i do it??

 

The 555 Monostable should work, either CMOS or conventional. There really isn't much difference between them.

 

I breadboarded this circuit

I am using a 50k resistor and 100uF capacitor instead of 100k + 1M resistor and 220uF cap.. I get a timing of 5 sec approx but most of the time I power on the circuit, the output goes low immediately instead of going low after 5 seconds.. I guess its a triggering problem.. how can I correct it??

 

You might try connecting NE555 pin 4 to pin 2 rather than VCC.

 

Let's see, 1.1 R(1E6) C(220E-6) = 242 seconds. You are talking minimum values, or you have a wiring error.

Pin 4 is not an input for monostable, it is only for enable or disable the timer.

You might also try the input conditioner I showed in my experiment referred to in post #2.

With a DVM you should be able to see the voltage change on Pin6 and 7, it starts at ground (normal), then slowly charges up to 2/3 of Vcc, then resets to ground.

 

Oh the ckt works now.. it was some problem on the breadboard.. Everything is fine now.. gives 5 second high, then goes low.. Thanks bill.. have to thank u a lot