Hi

Was wondering if someone may be able to help with the following. I have attached a circuit design for a one shot pulse to trigger an output either when the button is pressed or held in. What i need to be able to do is to trigger another pulse when one of the switches becomes normally closed. Any thoughts on how i may be able to do this?

I have attached the diagram below of what i have so far

Kind Regards

Dan

 

A couple of things.

1. Does the circuit do what you want now, before any changes? It appears that you want the circuit to energize a relay for a minimum amount of time, no matter how quickly a switch is pressed. If so, then see #5.

2. The 10 uF cap across the zener diode should be between Vin and GND.

3, All of the relay coil current goes through the zener diode. What is the diode's power rating?

5. The monostable pulse width is only 24 ms. That probably is too fast for the relay to respond. Assuming someone taps one of the switches very quickly, for how long do you want the relay to be energized?

4. The circuit will be much easier to discuss if you add reference designators to each component.

Separate from that, please, expand on your question. What do you mean by "becomes normally closed"? With the circuit you have, if you change one of the switches to a NC (normally closed) type, the other two switches will not operate the circuit.

ak

 

Hi Ak

Many thanks for your reply

1. Does the circuit do what you want now, before any changes?

The pulse of the circuit works exactly how i want it to, the problem comes when i need to be able to operate another switch when one is closed

2. The 10 uF cap across the zener diode should be between Vin and GND.

Rectified this

3, All of the relay coil current goes through the zener diode. What is the diode's power rating?

Breakdown voltage is 12V was trying to bring it down to 12v from 24v

5. The monostable pulse width is only 24 ms. That probably is too fast for the relay to respond. Assuming someone taps one of thw switches quickly, for how long do you want the relay to be energized?

Relay needs to be energised for 2 seconds maximum

4. The circuit will be much easier to discuss if you add reference designators to each component.

Separate from that, please, expand on your question. What do you mean by "becomes normally closed"? With the circuit you have, if you change one of the switches to a NC (normally closed) type, the other two switches will not operate the circuit.

Check annotations on this, hope this makes it a bit more clear

The switch calls for a new box of spares when the box is lifted up, this allows the last box to be called - however if i have a rack with more than one of the same part making a call one lane can run out before it is replenished therefore i need another switch to make a call in the meantime ( hope that makes sense)

 

2. Nope. The 10 uF decoupling capacitor goes between Vin and gnd on the 555. This compensates for the series impedance of the zener diode, filters power supply noise, presents a low source impedance to the 555's internal circuits, and acts as a current reservoir for when the relay is energized.

3. That's the zener voltage. I'm asking about its power or current rating. If the relay coil craws 200 mA, that is 2.4 W of heat in a 12 V zener diode, way more than a "standard" part can handle.

4. Reference designators are identifiers for each component. For example, resistors would be R1, R2, etc. A schematid needs both the component values and reference designators. If I recommend changing a 1 K resistor to 22K, which one am I talking about?

4a. Also, include pin numbers on ICs.

5. I recommend changing a 1 K resistor to 22 K. This will set a maximum ON time of approx. 0.5 s, something any relay can work with. Guess which resistor.

To keep the switches from overriding each other, you need three independent input stages driving the one 555. Each switch has a pullup resistor to the +12, a series capacitor, another pullup resistor, and then a small signal diode (1N914, 1N4148, etc.). Cathode toward the switch, anode toward the 555. All three anodes are connected together to the 555 Trigger input and its pullup resistor.

ak

 

I have attached the diagram below of what i have so far

Kind Regards
Dan

The 10uF capacitor connected to the trigger pin is backwards. The + end should go to the trigger.
Keith

 

Perhaps you just came here for a quick solution, but I would recommend that you put a little effort in learning how to solve a problem by analysis, and not by trial and error.

When you have such a problem, the best way to solve it is to actually to describe the problem by means of a boolean expression.
There are many web tutorials on how to do this, including one on this very site:
https://www.allaboutcircuits.com/te.../converting-truth-tables-boolean-expressions/

That boolean expression can be solved with truth tables and/or karnaugh maps, applying either minterms or maxterms.
Again this website has many chapters devoted to that, for instance:
https://www.allaboutcircuits.com/textbook/digital/chpt-7/circuit-simplification-examples/

and other similar chapters in the vicinity.

 

Below is the LTspice simulation of the circuit with AK's suggestion (slightly simplified) for the multiple inputs:

 

Drew up this, then got called away before posting. First pass at a circuit with three independent switches. Each switch delivers a 50 ms pulse to the Trigger input, and the 555 delivers a 0.5 s pulse to the relay coil. Because of the isolation diodes, one switch can deliver a trigger pulse even if the other two are closed.

The parts values can be adjusted to fit whatever you have available. The schematic shows a CMOS 555, but to drive a relay coil you should use the bipolar version (LM555, MC555, NE555, etc.) It has a higher current output stage.

ak

 

Below is the LTspice simulation of the circuit with AK's suggestion (slightly simplified) for the multiple inputs:

Yeah, about that mod ... I've never been comfortable with tying multiple pulse-forming capacitors together, like C2, C3, and C4 in your sch. Couldn't they form a voltage divider when one goes low while the other two are high, preventing the Trigger input from going below Vcc/3? I suppose the large ratio of R1 to R4-R5-R6 reduces this enough, but still ... I'm more comfortable with the three extra resistors in there.

As always, nice sim.

ak

 

I suppose the large ratio of R1 to R4-R5-R6 reduces this enough, but still ... I'm more comfortable with the three extra resistors in there.

Yes, it's the ratio of the two resistors that prevents any problem.
The simulation gives a negative pulse at the TRIG input that goes down below 1V when triggered (below).

But if you want to add extra resistors, that's fine.

 

Here is an updated version of the circuit. It includes the 24 V power source.

Also, it uses a relay with a 24 V coil (ignore the part number shown), powered directly from the 24 V input power. This removes the coil current from the zener diode, allowing a much lower power device.

ak

EDIT: R8 should be 220 K, for an output pulse of approx. 0.5 s.

 

Here is an updated version of the circuit. It includes the 24 V power source.

Also, it uses a relay with a 24 V coil (ignore the part number shown), powered directly from the 24 V input power. This removes the coil current from the zener diode, allowing a much lower power device.

ak
View attachment 193770

Brilliant, thanks for this most appreciated!

I'll give this a try and get back to you

Thanks for the replies all, you have been really helpful. wasn't expecting such an amazing response

 

Here is an updated version of the circuit. It includes the 24 V power source.

Also, it uses a relay with a 24 V coil (ignore the part number shown), powered directly from the 24 V input power. This removes the coil current from the zener diode, allowing a much lower power device.

ak
View attachment 193770

Not sure if i have something wrong, just checking over but the output to the relay seems to stay on for 30 seconds

 

Not sure if i have something wrong, just checking over but the output to the relay seems to stay on for 30 seconds

My mistake found the issue!

 

It calculates out to 0.24 s, which is a mistake. R8 should be 220 K, for an output pulse of approx. 0.5 s.

ak