just built nimh charger circuit but led will not light!

Hi, I've just built the following circuit on the breadboard

http://circuitdiagram.org/automatic-nimh-battery-charger-circuit.html

I've got 9.3 v on the top rail, 3.9v on pin 2 of the 7805 and 7.9v on pin 3. The transistor on the right has 0.68v on its base and the one on the left has 0.11 on its base. The circuit gives the required output ie approx 180ma using approx 26 ohm resistance. However the led never lights even though the batteries are discharged. Is anyone able to shed any light on this? No pun intended! Thanks!

That circuit looks like rubbish to me. Have no idea what it is intentions are. The "right" transistor is always on so the left one is always off (no LED ever lights). IMHO, the "engineer" who designed this never built it up and forgot to notice that the quiescent current from the 7805 is always flowing through the right transistor.

I cant open the link.

Here it is

bountyhunter said:

IMHO, the "engineer" who designed this

Click to expand...

Who said it was an engeneer, looks like it was an elementry school student to me.

A "better" charger would be to simply have a current limiting resistor in series with the battery and connecting it directly to the 7805. This has no way of shutting itself off though so it could easily damage the battery if not closley monitored. There would also be no way to connect an LED to the circuit to have it indicate the chaging status.

If you want a reliable battery charger for a NiMH battery, look into the MAX712 or MAX713. Those both have a status pin that can drive an LED while charging, and it is actually smart enough to charge the battery properly.

Dragon Tamer said:

Who said it was an engeneer, looks like it was an elementry school student to me.

Click to expand...

I was being polite.

But its on the web it must be right!

4pyros said:

But its on the web it must be right!

Click to expand...

If only it were ever that easy...

Dragon Tamer said:

If only it were ever that easy...

Click to expand...

If it was I would no longer have to pay for anything and I would be rich! But I also might get probed by aliens.

Almost every circuit on INSTRUCTABLES doesn't work.
One didn't work then it was copied by a 10 years old kid and of course it also doesn't work.
It uses the base-emitter of a TIP31 power transistor to short the output of an amplifier then it uses the transistor (without current-limiting) to blow up the LED.