I've connected things as you describe and am getting a clean signal from the emitter when it is not connected to the input on the BOB. as soon as I connect to the BOB it drop's to 1.5v. Just to clear up I'm not doing anything stupid on my part, I taking 5v from a pin on my BOB (next to pin 10 on BOB) to give 5v to the sensor and to the collector on the transistor.
Hi Dale
No worries. We'll get there.
It may be that the 10k pull-up resistor in the OPB916B cannot send enough base current into the 2N3904 to pull the emitter any higher than 1.5 V, but I find that rather hard to believe if everything is per the book. You see, with 1.5 V on the emitter that's 2.1 V on the base, or 2.9 V across the 10 k pull-up. That puts ~0.29 mA into the base. Allow a typical beta of 100 and you get 29 mA out the emitter - although most 2N3904 transistors should do better than that. Anyhow, 29 mA through 4.7 k should give well over 100 V drive! Odd, very odd, and not believable.
One possibility is that the front end of the buffer chip has been damaged, such that it can effectively short out any input current. (They all run on smoke: let the smoke out and they don't work any more.) You can check this by connecting the input pin to +5 V on the BOB and measuring the voltage at the pin. It should be of the order of 4.1 V. If it is much lower than this then I would suspect damage to the input buffer chip.
Fortunately, all the chips on the C11 appear to be in sockets, and they can be easily replaced. Spares can be bought through CNC4PC or through many electronics stores (Tandy?). But first, do the check with a 1 k resistor and let us know the results.
Cheers
Roger Caffin
Guest
