One NE556 works, another doesn't

[lavenatti]

I have a ST brand NE556N I've used to construct a missing pulse detector (see attached). It works just fine when using the ST IC. I stick something in between the LED emiter and the NPN optical transistor to temporarily break the beam and the output LED comes on for a second or so and turns back off. When I switch to a another brand of 556, I've tried NE556 from Samsung and an LM556CN from National - neither circuit triggers. The output pulse train to the LED works fine, I checked with a Oscilloscope. The second timer can also be manually triggered with a negative pulse (touched to ground) and seems to work just fine. It just won't trigger with the same input.
Any suggestions?

 

[alec_t]

Can you post the schematic?

 

[ClydeCrashKop]

Do you have / need a pull down / up resistor so it isn't just open when the beam is broken?

 

[lavenatti]

Here's a .bmp of the circuit.

I think it needs more than a pullup/down resistor. I'd be happy if that was all it was though.

 

[JimB]

A sea of white dots on a black background is not my favoutite, but I suggest that you put a pull-up resistor from RST-B to +Vcc, 10k should be OK.

JimB

 

[4pyros]

Your input from the sensor is AC coupled.
You need a pullup on the trigger input and maybe a pull down too. Try 1ooK to 10K.

 

[alec_t]

As for why one brand of 556 works and another doesn't, I suspect any existing internal weak pull-up resistance varies between brands.

 

[lavenatti]

I tried the resistive voltage divider and had no luck. Using a transistor (Q2) to ground the input worked, the diode (D3) eliminated a little pulse I was getting without it. The cap that previously fed the input signal to pins 8 and 10 is gone.

Thanks everyone for your help.