troubleshooting nightmare

[tarantulataramasalata]

hello
please help - i am trying to troubleshoot a circuit but I am really stuck.
I have a pic chip outputting to 2 4511bs, and then to a pair of 7seg leds for counting up to 99, powered by 2 3v lithium coin cells.

I have checked the circuit a squillion times, everything is wired properly and there's no shorts. I have replaced all chips and the 7seg displays work.

however, as soon as it is turned on, the voltage across the cells drops to about 3v, which implies that there is a problem somewhere, but I have run out of ideas where to look.

however, one of the 7 seg leds works absolutely fine, the other doesn't. the pic chip doesn't seem to be outputting on that side.

any help you can offer, as ever, very appreciated

 

[Rickharris]

Replace the coin cells temporarly with AA batteries and see if this makes a differance.

The voltage will drop under load - if the coin cells can supply the current they will drop the voltage.

 

[Technical]

When your display shows, for instance, 88 you will have 14 LEDs on at the same time. If each one takes, for instance, 10mA (it may be more) this is 140mA, far more than coin cells were ever designed to give out.

This will lead to the voltage drop and the battery draining very quickly.

 

[tarantulataramasalata]

hmmm...thanks technical.

problem solved - someone had put a 4510 into the 4511 drawer! It works fine now - and the 3v lithium coin cells seem to drive the 7 segment displays fine. don't know how long they'll last tho

thanks again