How to bypass the timing of 555 timer to drive output low when required?

Thread Starter

redgear

Joined Oct 17, 2019
136
I use a 555 timer as a WDT for the mcu in my circuit. The mcu has to send a trigger every second other wise the circuit timeouts and goes low after waiting for a 1 second.

I would like to drive the output low immediately bypassing the 1s delay when a switch is opened. Is it possible?
 

crutschow

Joined Mar 14, 2008
25,264
I would like to drive the output low immediately bypassing the 1s delay when a switch is opened. Is it possible?
Use a 2-input AND gate (e.g. CD4081) at the 555 output with a pull-down resistor at one input (e.g. 10k) that's pulled high by the closed switch.
 

Thread Starter

redgear

Joined Oct 17, 2019
136
Use a 2-input AND gate (e.g. CD4081) at the 555 output with a pull-down resistor at one input (e.g. 10k) that's pulled high by the closed switch.
Thank you. I could even remove the code to check if the switch is closed since it will be taken care of by the AND gate, am I correct?

Bringing the reset line of the 555 low will drive the output low, and reset the timing.

Connect the reset pin to VSS with a resistor and connect the pin to VCC with the switch.
Thank You! Cool idea. With this too I can remove the code to check for switch state....
 

crutschow

Joined Mar 14, 2008
25,264
The reset will work if the switches are NC types, but using the TR input for the heartbeat won't, since the 555 is insensitive to the TR input after it goes low.
The 555 internally is basically a level-triggered RS flip-flop, so once it's been triggered to the on-state, it will always time out until the THR (reset) input is triggered, which then resets the capacitor and returns the output to zero (see first attachment).
1596567950317.png

You need a retriggerable circuit. such as below. that resets the capacitor to zero for each input pulse:
The input is capacitively coupled so that whether the input hangs either high or low, it will still trigger an output error.

1596593070210.png
 
Last edited:

Thread Starter

redgear

Joined Oct 17, 2019
136
The reset will work if the switches are NC types, but using the TR input for the heartbeat won't, since the 555 is insensitive to the TR input after it goes low.
The 555 internally is basically a level-triggered RS flip-flop, so once it's been triggered to the on-state, it will always time out until the THR (reset) input is triggered, which then resets the capacitor and returns the output to zero (see first attachment).
View attachment 213993

You need a retriggerable circuit. such as below. that resets the capacitor to zero for each input pulse:
The input is capacitively coupled so that whether the input hangs either high or low, it will still trigger an output error.

View attachment 214015
Oh, i guess then its better to use a logic gate as you suggested in #5. What do you suggest?

Thanks
 

crutschow

Joined Mar 14, 2008
25,264
i guess then its better to use a logic gate as you suggested in #5
I said nothing about a logic gate in my post #11.

Did you understand what I said about the problem with the heartbeat input?

I suggest you use the circuit I posted, with the addition to the Reset input that you posted.

(The "In" in my circuit is the heartbeat input.)
 

Thread Starter

redgear

Joined Oct 17, 2019
136
I said nothing about a logic gate in my post #11.

Did you understand what I said about the problem with the heartbeat input?

I suggest you use the circuit I posted, with the addition to the Reset input that you posted.

(The "In" in my circuit is the heartbeat input.)
I said in your the post #5 not #11. You suggested that I use a 2 input AND gate with one input being the output from 555 and the other being the switch.

I was interested in that solution as number of components were low.

Thanks!
 
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