HI, i'm building a keypad circuit to switch a relay. I got the circuit from

http://www.mitedu.freeserve.co.uk/Ci...Alarm/pad4.htm

I don't have a proper bench power supply so am using aa batteries in series to give 7.5-9v. When i start testing it works for a bit and then tranisitors 4 and 5 seem to stop working i.e. Q5 wont turn on when E is pressed, Q4 wont turn on when there's an output from pin 10

Does anybody know why this would be?
I plan to put it in a car. Will the car battery fry Q5 straight away?

Is there another transistor that is more durable than BC547? Or are they usually ok and it's just something i'm doing??

Cheers for any advice


Also, i had to remove Q1 and Q2 cos they seemed to reset the cascade whenever they were pressed, even if it was at the right point in the sequence. I ran the circuit on a circuit simulation software and it didn't work with Q1 and Q2 plugged in. Any ideas??

cheers :?

 

Have you paid attention to the pinout of the BC547? It is the reverse of most 2Nxxxx transistors.

 

Yeah. They are correctly placed.... :?

 

I would check your supply voltage when you press a key.

 

a BC547 is a NPN transistor and requires biasing on its base,I assume your going to have 12vDC in the car,the usual biasing for switching is 560 ohm 1 watt resistor the BC series of transistors are pretty hardy,if you feel the transistor, while on load and it gets a bit to hot, I suggest a shunt resistor at the collector,trial and error is required here,but when the transistor remains just warm on load it should be ok.a schematic diagram would be a help to answer you properly.

__________________
leon

 

hi,

have you handled your CMOS device correctly assuming the transistors are okay.

__________________
\"NONE BUT THE BRAVE DESERVES THE FAIR\" ~Shakespeare~