Hi,
I have an input pulse trigger signal (from LED) that first triggers 5 times (high/low 4.5/2.5v - 1sec interval-) and after that it stays low for 30 sec’s and than goes high again. I need to create a pulse trigger signal out of this trigger that only fires high when the input pulse stays low longer than 1 sec.
So It has to skip the short (1 sec interval) pulses. The final output trigger stays high as long as the long input (long ± 30sec) pulse stays low.



Is it possible to make such an electronic circuit ?

It seems it can be done by an RC filter with a zener diode as threshold
When the capacitor is charged by the 1 sec pulses it does not reach the zener threshold
The 30 sec pulse charges the capacitor fully so the zener diode starts conducting. With a
logic buffer in the end the signal can be shaped 0 and 1.

The input pulses have a negative value so I do not know how to design the circuit and I also
have no knowledge of the resistor, capacitor and zener values.

please help

Henk Wijtman
Rotterdam
Netherlands

 

Yes, such a circuit is possible. Would you like to draw your idea for comment first?

 

That is called a missing pulse detector, which is a form of a retriggerable monostable multivibrator circuit.

ak

 

DickCappels,
Thank you for answering. I have drawn a very clumsy idea because I have very little knowledge of electronics. It's assembled from circuits on the internet and partly my minimal electronic logic.



And AnalogKid a will look up the 'missing pulse detector' ?
Please comment

 

I offer this circuit. There is no guarantee that it will work for you. You need to play with the component values.

I need to know the resistance of the relay and current and voltage required to operate the relay.

I also need to know how much current is supplied by the input signal.
Is the timing 0.5-second on and 0.5-second off?

 

MrChips
Thanks, Timing is 1 sec on, 1 sec off.
Other values (resistance, current) I have to measure.
Relay is 5 volt

 

Some new values to try:

R1 = 1kΩ
R2 = 750Ω
C1 = 4700μF

 

MrChips
I will try the circuit and keep you posted. Could you explain the circuit to me so I am learning something too.
This is the relais I am using.

 

MrChips,
I think this is a link of the specifications of the relay
https://docs.rs-online.com/8b2b/0900766b81398112.pdf

 

Sorry, I am busy all day. I will respond when I have some time.

 

View attachment 241468

Not a lot wrong with this circuit. You could use 1uF/1MΩ, and maybe put 1k in series withe schottky diode to limit the current. You may have to adjust the values to get it to work perfectly. If you use 74LVC14 instead of 74HC14 or 40106 the thresholds would be a bit more predictable.

 

Hi,
I have an input pulse trigger signal (from LED) that first triggers 5 times (high/low 4.5/2.5v - 1sec interval-) and after that it stays low for 30 sec’s and than goes high again. I need to create a pulse trigger signal out of this trigger that only fires high when the input pulse stays low longer than 1 sec.
So It has to skip the short (1 sec interval) pulses. The final output trigger stays high as long as the long input (long ± 30sec) pulse stays low.

View attachment 241464

Is it possible to make such an electronic circuit ?

It seems it can be done by an RC filter with a zener diode as threshold
When the capacitor is charged by the 1 sec pulses it does not reach the zener threshold
The 30 sec pulse charges the capacitor fully so the zener diode starts conducting. With a
logic buffer in the end the signal can be shaped 0 and 1.

The input pulses have a negative value so I do not know how to design the circuit and I also
have no knowledge of the resistor, capacitor and zener values.

please help

Henk Wijtman
Rotterdam
Netherlands

Here is an alternative circuit.
This is a Missing Pulse Detector as mentioned in post #3.
The input signal responds to logic level of 4.5v=high, 2.5v=low. If no input pulses are detected, the relay remains energized. If pulses are detected, relay becomes de-energized.

 

The relay has a DC coil resistance of 167Ω and operates at 30mA @ 5V.

I used a 2N3906 PNP bipolar transistor because it matches your 5V/2.55V input nicely.
With 5V supply, the transistor needs greater than 4.5V to turn off and lower than 4.3V to turn on.
To drive the relay with 30mA we want to turn on the base with about 3mA.
With the values of R1 = 1k and R2 = 750, the base current is about 1mA which should be ok.
The value of C1 = 4700μF is higher than I would like but 1-second intervals don't give us much choice.

I have shown simulations with two different time intervals.
The first simulation is with 1-second inputs ON and OFF (A - green trace).
C1 holds the voltage above 4.3V (B - yellow trace) and keeps transistor Q1 in the off state.
C - cyan trace is the voltage on the relay.



In the second simulation, the signal is on for 10 seconds and off for 10 seconds (A - green trace).
The voltage on the base falls below 4.2V (B - yellow trace).
The voltage on the load R3 is switched on at 4.8V (C - cyan trace).

 

just a thought: would a resistor in series with the base be useful for when power is first applied?

 

Hi you all,
I want to thank you for all answers I've got for 'my' electronic problem. I am a very electronic newbe (even have to lookup the pinout of a transistor) but through your answers I am learning a lot. I am going to experiment now and let you know the result (or more questions ?).
Thanks again
Henk

 

McChips,
Special thanks to you for taking the time to explain extensively. Just read it and I understand your circuit & simulations.

 

(even have to lookup the pinout of a transistor)

It's important that you do. Even components with the same type number might vary from one manufacturer to another.

 

YES!! Just put the circuit of MrChips in test together and it works like a charm. Gonna build it into the alarm now. I bought the smallest 4700uf (16 v) I could get and hope it wil fit inside the enclosure.
Thanks again

 

just a thought: would a resistor in series with the base be useful for when power is first applied?

Good point.

There is a large current flowing through the emitter-base junction in order to charge the capacitor from 0V to 5V on first power on. Placing a 100Ω resistor in front of the base would reduce the current.

An alternative is to move the capacitor C1 and place it across R2, keeping the positive terminal at +5V.

 

Hi,
I just implemented the latest modification to the circuit (moved the capacitor across R2). But now the the relais already reacts on the last 2 of 5 pulses. I think I have to make an adjustment to one of the resistors. But which one and how much.