Timer problem.....

[Rescue1]

Hello again everyone.I made a timer circuit based on the 555 in monostable mode to be triggered from a paging device that gives a 3.2volt(high) output.The output of the 555 goes to an NPN transistor,which in turn drives a ground to a dpst 12volt relay (a diode is placed across the coil).In this format the circuit works great.However,the paging device has a 3.2volt output when activated lasting approx. 2 seconds,and the 555 needs a low,or ground to activate it.I wired up a NPN transistor to the pager output to convert the signal.The base has a 4.7k resistor to ground,then the pager output connects to the base,emitter to ground,and collector to the 555.The circuits works when the pager is triggered,turning the relay on for approx. 25 seconds as required.However,the circuit is very touchy,especially when a RF source is brought into the same room,like a 2-way radio.It will trigger the circuit even over 5 feet away.This does not happen when the (pager side)NPN transistor is disconnected from the 555.I have tried different resistors to bias the transistors and 555 inputs but it still is touchy.Any help would be appreciated,or even a new circuit.Thanks for the help..... :?

P.S.-I don't expect the circuit to be completly RF proof as even my fluke meter gets loopy when RF is near,but having it triggered in the same room to not acceptable...... :roll:

 

[Rescue1]

Any help would be appreciated.Would posting a schematic produce some reply's??I will try and figure out how to do that.....

 

[Rescue1]

I'M desperate here.I have tried many different ways to do this.When I change resistors around I can somtimes get it to be more immune to the rf,but then when the 555 is activated it will not shut down.I am pulling hair out here,calling all experts,PLEASE HELP.... :?:

 

[JimB]

OK, try these:

Keep the wires short.
Where you have two wires, ie from the pager to the circuit, twist the wires together.
Put a 4k7 resistor between the pager output and the transistor base (put this resistor close to the transistor), put a 4k7 resistor from the transistor base to emitter (ground).
Connect a 0.01uF cap from base to emitter on the transistor.
Connect a 0.1uF cap across the supply at the 555.

JimB

 

[Someone Electro]

also try to put it in an metal casing and ground it

 

[David Bridgen]

Try this.

 

[Rescue1]

Thanks.. ....A combination of all these suggestions worked well.I had to tweak the resistor values because sometimes the timer would not shut down,but I finally got it.Boy I have made many other more complicated projects,but this one proved to be the most annoying.Maybe because I was in a hurry,thanks again.... 8)