Hello Everyone,

We have a rather peculiar issue we're trying to explore. We are using a red laser pointer to illuminate a photo-transistor, which then feeds into the #2 pin of a 555. When the beam is broken, #2 goes low and the #3 pin activates a relay which powers up a 24V solenoid. We use ping-pong balls to break the beam. We currently use this version below, which works every time the beam is broken:



The problem is, as long as the beam remains broken the relay continues to feed 24V to the solenoid (which also has a diode across the terminals).

Right now, there are 50 solenoids, each fed with its own PCB. I know the solenoids can handle the current, but to stress them as little as possible I just want the relays to activate ONCE when the beam is first broken, and then turn off even if a ball happens to still be blocking the beam.

If a ball bounces in, breaks the beam, and then bounces out, great. Relay is still off. If by chance a ball breaks a beam, bounces out, then bounces back, again no big deal - the relay will only activate twice in total. We can live with that. We just don't want to chance 50 solenoids all on for perhaps 10 minutes at a time.

We did some research and prototyping, and came up with the following:



This one has a PNP transistor, etc added to the original circuit, which is supposed to trigger on a falling edge. It seems to work, but it's doing peculiar things, too, which we'd really like to understand. These are:

1. For about the first 15-20 seconds after power-up, nothing responds to a beam break. However, this doesn't always happen.

2. After a beam is broken and the relay activates, one of 2 things can happen: (a) again, the circuit becomes unresponsive for about 15-20 seconds when the beam is interrupted a second time, or (b) the circuit seems to work just fine and the next time the beam is broken the circuit activates the relay as expected. (I do realize there is a deliberate non-responsive time built in by the RC circuit, but that should only be about (100uF x 2.2k x 1.1) = 0.24 seconds, not 15-20.) Again, I don't seem to get consistent results.

It could be there is a poor connection somewhere, but I'm wondering if possibly there is a flaw in the design? This was a bit ad-hoc, since we couldn't find anything out there that dealt specifically with using a photo-transistor.

Any insights gratefully received!

Thank you in advance.

 

The circuit should work without Q2, R3 and R5.

Try just using a .1uf cap and a 10k pullup where R2 is. (these values may need to be adjusted)

You can leave D3.

 

Why are you showing the 5V from a 555 activating a relay with a 24Vdc coil?
I'm surprised it even works.

 

You simply have the wrong type of monostable. If you use a non-retriggerable monostable such as the 74HC221 it will do exactly what you want.

 

Why are you showing the 5V from a 555 activating a relay with a 24Vdc coil?
I'm surprised it even works.

If I understand your question correctly, the relay is a JQC-3F, and it only needs 5V switching voltage to operate. You can get them with a 12V or even 3V. They seem to be everywhere.

 

The circuit should work without Q2, R3 and R5.

Try just using a .1uf cap and a 10k pullup where R2 is. (these values may need to be adjusted)

You can leave D3.

Thank you very much. When I get in Monday I'll give it a try and report back.

 

You simply have the wrong type of monostable. If you use a non-retriggerable monostable such as the 74HC221 it will do exactly what you want.

Thank you for the information. Could you expand on that? We are real beginners at this. Thanks!

 

Thank you for the information. Could you expand on that? We are real beginners at this. Thanks!

The old Fairchild datasheet illustrates it nicely on page 5 (Texas Instrument's and NXP's datasheets are not as clear)
You can see that the output pulse is exactly the same length regardless of how long the input pulse is, or if two input pulses occur before the output pulse has ended.

 

If I understand your question correctly, the relay is a JQC-3F, and it only needs 5V switching voltage to operate. You can get them with a 12V or even 3V. They seem to be everywhere.

I know they can have different coil voltages, but the schematic shows one with a 24V coil.
If that's not correct then please correct it on the schematic.

 

You simply have the wrong type of monostable. If you use a non-retriggerable monostable such as the 74HC221 it will do exactly what you want.

The second schematic with the added transistor has a differentiator capacitor on the trigger input which makes it non-retriggerable.

 

The second schematic with the added transistor has a differentiator capacitor on the trigger input which makes it non-retriggerable.

Yes - that works too.
HC221 has some advantages, such as a choice of both positive and negative going inputs and outputs (all of which can be added to a 555 with extra circuitry), and you get two in a package, but it can't drive a relay directly.
It's very rare that there is one and only one way of accomplishing something!

 

Thank you very much. When I get in Monday I'll give it a try and report back.

As suggested by ElectricSpidey.
I changed the values of C1 and R1 providing the same time constant but using a smaller capacitor.

 

It's very rare that there is one and only one way of accomplishing something!

Very true.
My goal is to look at the alternative ways, and use the simplest that still meets all the performance requirements.

 

As suggested by ElectricSpidey.
I changed the values of C1 and R1 providing the same time constant but using a smaller capacitor.
View attachment 330077

Thank you all very much. I just set up the circuit as above, and unfortunately it does not appear to work. I hooked up an oscilloscope to 3 different points:

P1 - between the photo-transistor and Ground. Starts at approximately 5V. When beam is interrupted, Voltage plunges to zero. This seems correct.

P2 - one probe on Pin #2 (TR) the other on Ground. This ALWAYS shows 0V, regardless of whether the beam is interrupted.

P3 - one probe on Pin #3 (Q) the other on Ground. This ALWAYS shows 5V, again without regard to the beam state. This does not surprise me, since Pin #2 is permanently low.

I did notice some things, though. Electricspidey suggested .1uF on C2 and 10k on R2, whereas the circuit here suggests 10nF, or 0.01 uF, and 100kOhm rather than 10kOhm. Could this be why Pin #2 seems permanently low?

 

I know they can have different coil voltages, but the schematic shows one with a 24V coil.
If that's not correct then please correct it on the schematic.

I think I see the issue now. The verbiage is what the KiCad symbol has as default. I will fix it to avoid confusion.

 

P2 - one probe on Pin #2 (TR) the other on Ground. This ALWAYS shows 0V, regardless of whether the beam is interrupted.

That tells me that R2 is open or not connected correctly, check the wiring. Pin #2 should read 5 volts when the beam is not interrupted.
Pin #2 should drop to appx 0.9 volts briefly when the beam is broken. This is set by the voltage divider of R3 and R2.
The voltage on pin#2 must drop below 1/3Vcc or 1.67v to trigger hence why R2 is set at 100K.

 

That tells me that R2 is open or not connected correctly, check the wiring. Pin #2 should read 5 volts when the beam is not interrupted.
Pin #2 should drop to appx 0.9 volts briefly when the beam is broken. This is set by the voltage divider of R3 and R2.
The voltage on pin#2 must drop below 1/3Vcc or 1.67v to trigger hence why R2 is set at 100K.

Progress! You were right, I miswired one of the resistors. I now do see a voltage drop on #2. However, it's unfortunately tiny. I can't even measure it very well, as it passes by quickly. But I can say it's much less than 1V.

 

Does the circuit work now?

 

Does the circuit work now?

I'm afraid not. I do not think the voltage drop at #2 is enough to trigger the output.

But thank you for your continued interest!

 

With the cap the voltage must drop rapidly. Might be the response of the opto transistor.
What values are you using for R1 and C1?