Triggering 555 timer

  • Thread starter djkross
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  • #1
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hey guys... i'm having some trouble trying to figure out how to trigger a siren i'm working on. i have an astable circuit setup with the output of the monostable 555 connected to the output of the astable 555s to trigger the circuit. i wanted to trigger the circuit using a PIC microprocessor but a teammate of mine says i can't do that. so far, the only way it's actually working is by grounding the trigger pin of the monostable 555. is there a chip or a circuit that i could add to this existing one that will be connected to the trigger pin of the monostable 555 and could have a signal sent to it by the PIC that will ground its output and thereby grounding the trigger pin of the monostable 555, which will get the result that i'm looking for?
 

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  • #3
Tom.G
Science Advisor
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I checked the data sheets for the PIC16F84A and the Texas Instruments LM555 and don't see a problem.

If the I/O port of the PIC is set as an Output and the supply voltages are compatible, I see no reason it wouldn't work. Ideally the PIC and the 555 Monostable should be driven with the same power supply, +5V and Gnd.

Be sure there are power supply bypass capacitors installed fairly near the chips.

Make sure your program sets the PIC output pin you are using to be Output, not Input. The Trigger pulse from the PIC to the Monostable should be normally high with a low pulse at least 1uS duration, perhaps longer if you are using a CMOS version of the 555. Since the PIC can operate with a 20MHz clock, you program will probably need a time delay to set the pulse duration

The Trigger pulse should also be shorter than the 555 output pulse.

If you have an Oscilloscope available, check for a pulse on the PIC output pin.

If all that fails to fix it, have someone else check your wiring against the schematic. Next would be to either replace the PIC and 555 or try them in another working circuit to see if they are still good.
 
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