LED and relays controlled by 555 timer

Thread Starter

edgeb

Joined Jun 18, 2020
23
Hi folks

Electronics is not my forté (as in, I have zero experience with it) so bear with me if my model sounds lousy or my language is not accurate.

Anyway, I’d like some advice on the following please. I need to design a circuit which achieves the following:

1. Receive 5v incoming at 500hz from a flow sensor
2. Output that to around 3hz for a warning LED to flash at a decent frequency
3. The 500hz (or 3hz) signals should also close a relay which activates a pump
4. However, when the incoming signal switches to constant 5v, the LED should stop flashing (Constantly on) and the relay should open, stopping the pump

So far I’ve been doing the following:

1. A 555 timer configured as frequency divider drops the incoming 500hz frequency to around the desired 3hz
2. A second 555 in monostable mode receives that 3hz signal (which has very high duty cycle and therefore makes an LED on the output appear constantly lit) and outputs a 3hz signal with lower duty cycle - thus an LED will flash at a human-readable rate
3. When the source goes from 500hz to constant 5v, the LED turns solid.
4. A third 555, also monostable, fed by the output of the second 555, takes that 3hz signal and increases the duty cycle significantly, outputting that to a relay which is held open by that 5v output. Note: I don’t want to use a pulse relay because it’s important that the pump stop if signal is lost for whatever reason.

This model I’ve made works (lights flash and relays open accordingly) but every now and then the pulse from the third 555 to the relay ends (no matter how long the output is, it’s always a pulse) and the relay opens for a split second, before closing again with the next pulse.

So my questions are as follows:

1. is this a sensible design or are there more elegant ways to achieve the desired result?
2. Should I be using other components instead of 3 x 555s?
3. How do I prevent the output from the third 555 from opening the relay occasionally while there’s still a pulse being fed into the system?

I hope this explanation makes sense!

Thanks

Edge
 

jpanhalt

Joined Jan 18, 2008
9,754
You might be able to avoid the second 555 using a simple inverter (NOT) gate:
1592833735331.png

Then, the LED would be mostly off with short flashes.

What causes the input to go from 500 Hz to DC? Is 500 Hz "normal" and DC abnormal?
 

Thread Starter

edgeb

Joined Jun 18, 2020
23
You might be able to avoid the second 555 using a simple inverter (NOT) gate:
View attachment 210435

Then, the LED would be mostly off with short flashes.

What causes the input to go from 500 Hz to DC? Is 500 Hz "normal" and DC abnormal?
The fuel flow sender transmits 0v/5v at 500hz when fuel is flowing. When it runs out of fuel it’ll send constant 5v.
 

jpanhalt

Joined Jan 18, 2008
9,754
That sounds like the job for a missing pulse detector. That's a standard 555 circuit:
1592835887059.png

The original source (AN170) is no longer on the Philips site. I will upload it here.
 

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Thread Starter

edgeb

Joined Jun 18, 2020
23
That sounds like the job for a missing pulse detector. That's a standard 555 circuit:


The original source (AN170) is no longer on the Philips site. I will upload it here.
Thanks, I didn’t know that was a function of the 555!
 

gcalderon

Joined Jun 8, 2019
1
You can use a NE567 IC as 500Hz tone detector It is a Phase Look Loop (PLL) that can be tuned to 500 Hz signal from fuel sensor. Its output is a sustained level signal than can be used for activate the 555 oscillators, taking its "reset" (pin 4) input from the output of the PLL. So, you can control the LED as desired and the relay for pump control.
Have a good look from San José of Costa Rica, the "Pura Vida country"
 
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