Hi all - To start off, my circuit seems to work perfectly as intended but I'm experiencing random triggers on my 555 timer which ultimately closes my 40DA relay. A little context, the two signals, S- and S+ are from a piezo buzzer from a 5v, DC alarm clock. There's about a 1v potential created when the buzzer emits a sound. The output keeps a SSR closed for roughly 6 minutes to brew me some coffee in the morning.

I understand all the buffers may be unnecessary, especially the one after the differential amplifier but I had the spare op-amp on my IC and I wasn't sure if it could help or not. I haven't added the decoupling cap (C3) to the circuit quite yet but I'm optimistic that will help this issue and limit some noise.

My questions though, first, I've seen many circuits have the output of the 555 go to the gate of a transistor to drive their relays in similar circuits. Does this provide any benefit? My relay can switch at 3VDC (it claims) where the 555 provides a little over just that. Also, I've read about "flyback" diodes on the AC output of the relay, can someone explain maybe if this could be helpful or completely off-topic?


Here's my relay (https://www.amazon.com/gp/product/B07KNKYHBY/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&th=1)

Any help would be greatly appreciated. Please bare with me as I'm still an EE student and am learning. If I can provide anymore context I'd be happy to!

Joe

 

Also, the difference isn't exactly 1v from the buzzer emitting a sound. The 50% voltage divider provides a little over the 0.7v gate threshold when there's a sound. Reread my post and wanted to clarify that. I don't believe there's an issue with the left half of the circuit but, again, any overall critique is still appreciated.

 

There should be a resistor between the base of transistor TR1 and OUT2 of IC2. I would suggest a value of at least 50kΩ, you could then add a capacitor between the base of TR1 and GND to provide crude filtering. A 1µF capacitor may be sufficient to avoid nuisance triggering of the timer circuit.
You might need to provide a little more gain via IC2/3 to get better results.

 

I'm still an EE student and am learning.

Is that how they're teaching students to draw schematics? Yours in unreadable because you used pinout order logic symbols instead of symbols that convey functionality.

If you use connection dots, you don't use "humps" for wire crossings.

EDIT: This is what I transcribed from your wiring diagram:

Two issues off the bat: you don't have current limiting resistors for Q1 and K1. The opamp isn't likely to damage the transistor, but the timer will likely destroy the diode in K1.

What is the nature of the input signals?

I've read about "flyback" diodes on the AC output of the relay, can someone explain maybe if this could be helpful or completely off-topic?

Only required for mechanical relays with coils.

 

I don’t think you can blame joeyhodson32 for the way the software package lays out the circuit. The two LM358 ICs are shown with the IC pin-outs rather than a pictorial depiction of op-amps – which makes reading the circuit harder.

I prefer the use of humps for wire crossings, otherwise you might need to scrutinise the diagram closely to work out which crossing wires are joined.

Another observation on the circuit design, the output of the 555 timer should include a resistor to limit the opto diode current – otherwise the timer will supply 5V at up to 200mA to the diode.

 

Something else to consider in your circuit design; if the 555 timer pin 2 is held low for longer than the timed period, then the timer output will remain high until the timer input goes high. So the AC load will remain energised beyond the timed period if the alarm clock continues to sound.

 

Yours in unreadable because you used pinout order logic symbols instead of symbols that convey functionality.

I appreciate the constructive critisim. We of course work with functional symbols in university yet I haven't made a lot of schematics for my own projects yet. This is good to know for future reference, I get using the pinout symbols can make it a bit harder to read and analyze.

Two issues off the bat: you don't have current limiting resistors for Q1 and K1. The opamp isn't likely to damage the transistor, but the timer will likely destroy the diode in K1.

Any values you would suggest? I can do some research in the meantime.

What is the nature of the input signals?

Forgive the jargon I use to describe the signals but I would consider them very volatile. I don't have exact values at the moment as I can't probe right now, but the values of both + and - seem to bounce around quite a bit.

 

Something else to consider in your circuit design; if the 555 timer pin 2 is held low for longer than the timed period, then the timer output will remain high until the timer input goes high. So the AC load will remain energised beyond the timed period if the alarm clock continues to sound.

I have experienced just this. Thank you for explaining exactly why this is occurring. Do you have any suggestions for correcting this?

 

I prefer the use of humps for wire crossings, otherwise you might need to scrutinise the diagram closely to work out which crossing wires are joined.

Thanks for the reassurance. I thought it would help the readability of the circuit as well.

Another observation on the circuit design, the output of the 555 timer should include a resistor to limit the opto diode current – otherwise the timer will supply 5V at up to 200mA to the diode.

I'll definitely implement this into the circuit, any suggestions on value? I'll be researching and looking more into this. I haven't worked with SSRs before and opto diodes.

 

See my post in the link (#7) for a circuit that will do what you want, stopping a low input extending the timed period.

https://forum.allaboutcircuits.com/...ail-safe-relay-555-timer.152685/#post-1312068

 

You need to check the opto diode spec for the normal forward voltage/current.

As an example, if the diode nominally requires 2V@10mA, then to reduce the 5V timer output to 10mA will require a series resistance of (5V-2V)/0.01A = 300Ω.

 

Any values you would suggest? I can do some research in the meantime.

The transistor had a 10k collector resistor, so you only need 0.5mA collector current for saturation. Assuming the output of the LM358 can only get to 2V, I'd use 20k.

The relay LED probably wants 20mA max. Assuming the timer output will only go as high as 3.5V and 2V for the LED forward voltage, I'd use 150 ohms. If the output is closer to 5V, you'd be near the maximum diode current.

What year are you in your studies? Calculating the resistor values only uses Ohm's Law and KVL.

I get using the pinout symbols can make it a bit harder to read and analyze.

Then you're a better engineer than me. I find them impossibly difficult to read. I create a proper schematic before attempting any analysis.

Forgive the jargon I use to describe the signals but I would consider them very volatile. I don't have exact values at the moment as I can't probe right now, but the values of both + and - seem to bounce around quite a bit.

What is the amplitude? What is the period? You have the trigger DC coupled to the trigger input, so the transistor needs to turn off before the one shot times out.

 

I agree that this is potentially a very educational subject. We need to give credit for making the thing work, but as we have already seen, there are many possible improvements.

Someone needs to ask if the common or ground for the buzzer circuit is connected to the circuit in the linked schematic!

Is there actually signal on both ends of the buzzer? With a 5V supply, it's common to drive just one end.
Are the DC voltages at both ends of the buzzer well defined when it is not buzzing? What are those voltages? What are the DC impedances?
Have you observed the buzzer drive signal? It might be useful to know the frequency of the buzz, so as to better filter out higher frequency noise.

It is important to have and understand the device data sheets.
I found a SSR, perhaps the correct one: https://www.digikey.com/en/products/detail/sharp-microelectronics/S216SE1/458287
https://media.digikey.com/pdf/Data Sheets/Sharp PDFs/S216SE2.pdf

Is the NE555 a real NE555. Or one of the newer CMOS versions that is "pin for pin", but different in important ways.

Just the way the circuit is constructed can cause intermittent firing. Especially when there are AC line / mains connections nearby.

It's very likely that a satisfactory circuit can be devised that dispenses with the op amps and the transistor. The 555 trigger input is high impedance, and will respond to a small voltage change. A passive band pass filter and DC bias network might connect the buzzer signal to this pin.

Ted

 

I remember "humps" for wire-crossings in schematics when I started a half-century ago, but seldom in schematics since then.
Dots are a lot easier to use in schematic construction, and I've never used humps.
Also I think just about all schematic capture programs used dots.
I'm comfortable with dots, so it's never a problem for me to see which crossing wires are connected.
Actually I find humps distracting, rather like potholes in the schematic.