Torben
Well-Known Member
Hi all,
I've got a beam-break detector working. The circuit below works, but I don't think it's a good circuit. I'm happy with the 555 (not saying it couldn't be improved); it's the transistor trigger section I'm not sure about.
The purpose is to cause pin 2 on the 555 to go LOW when the beam striking the phototransistor is broken. The 555 should pulse out for about 3 seconds when that happens. So far, this works fine. I'm just not sure how goofy the circuit is.
Here's the circuit:
**broken link removed**
It's all +5VDC. The far right + output is the trigger out, it actually goes to a resistor then an inverter into the counter.
What I need to know is this: if I'm reading my books right, an NPN should have the collector positive relative to the emitter...but in the above it seems it's the other way 'round. However, it demostrably works. What I can't predict with what I know is how reliable it is.
Here's my noob explanation of what's going on:
When PT1 is lit, it robs the base of T1 of its current by diverting it to ground, which causes T1 not to conduct, pulling the base of T2 to ground via R1. When the beam is broken, PT doesn't conduct and T1's base is free to go high via R2. Therefore T1 conducts, also pulling T2's base high, which brings the 555's pin 2 to ground, triggering the monostable.
T2 is just there as an inverter/buffer because it was for a piezo trigger as originally designed, but piezos turned out to be fiddly to position and fragile. Still good for simple drum triggers though. It also seems upside down.
Anyway, would I be OK leaving it like this? If not, would simply swapping in 3906s make it safer? I'm still researching, but at times in circles. Learning as a hobbyist has it plusses but no prof/mentor makes it...interesting.
Thanks,
Torben
I've got a beam-break detector working. The circuit below works, but I don't think it's a good circuit. I'm happy with the 555 (not saying it couldn't be improved); it's the transistor trigger section I'm not sure about.
The purpose is to cause pin 2 on the 555 to go LOW when the beam striking the phototransistor is broken. The 555 should pulse out for about 3 seconds when that happens. So far, this works fine. I'm just not sure how goofy the circuit is.
Here's the circuit:
**broken link removed**
It's all +5VDC. The far right + output is the trigger out, it actually goes to a resistor then an inverter into the counter.
What I need to know is this: if I'm reading my books right, an NPN should have the collector positive relative to the emitter...but in the above it seems it's the other way 'round. However, it demostrably works. What I can't predict with what I know is how reliable it is.
Here's my noob explanation of what's going on:
When PT1 is lit, it robs the base of T1 of its current by diverting it to ground, which causes T1 not to conduct, pulling the base of T2 to ground via R1. When the beam is broken, PT doesn't conduct and T1's base is free to go high via R2. Therefore T1 conducts, also pulling T2's base high, which brings the 555's pin 2 to ground, triggering the monostable.
T2 is just there as an inverter/buffer because it was for a piezo trigger as originally designed, but piezos turned out to be fiddly to position and fragile. Still good for simple drum triggers though. It also seems upside down.
Anyway, would I be OK leaving it like this? If not, would simply swapping in 3906s make it safer? I'm still researching, but at times in circles. Learning as a hobbyist has it plusses but no prof/mentor makes it...interesting.
Thanks,
Torben
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