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555 monostable on power delay

am.steen

Jul 10, 2023
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I made the circuit in attachment and it works As expected when connected to dc source
When try same circuit on transformerless power supply consists of capacitor - resistor - bridge - zener 12v
It is not triggered on power but you have to trigger it manually to work

Please help
 

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crutschow

May 7, 2021
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It is not triggered on power but you have to trigger it manually to work
Likely the bridge supply comes up more slowly then connecting it to the DC supply so the TR signal doesn't say low, long enough.
Try increasing the value of C1 to 10µF or greater (and/or increasing the value of R3).
 

AnalogKid

Jun 10, 2015
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There is no filter capacitor in your description of the offline power supply. Without one, the circuit is powered by rectified AC swinging constantly between GND and 12 V. That can't be good for circuit operation.

If you do in fact have a filter capacitor, then your post is an excellent example of why it is important to post a schematic of your circuit.

ak
 

Delta Prime

Jul 29, 2020
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Also.... Simulation or not it is good engineering practice.
(pin 5 )put a 0.01 microfarad capacitor to prevent fluctuation of your RC network.
Monostable or also called one shot. The capacitor on pin 5 will guarantee your output current pulse when you press the button manually or as you put it...when power is applied.
1689026159163.png
 
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am.steen

Jul 10, 2023
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Also.... Simulation or not it is good engineering practice.
(pin 5 )put a 0.01 microfarad capacitor to prevent fluctuation of your RC network.
Monostable or also called one shot. The capacitor on pin 5 will guarantee your output current pulse when you press the button manually or as you put it...when power is applied.
View attachment 59808
Likely the bridge supply comes up more slowly then connecting it to the DC supply so the TR signal doesn't say low, long enough.
Try increasing the value of C1 to 10µF or greater (and/or increasing the value of R3).
Increasing c1 to 10 or 100uf works As circuit triggered at power on but when power off and on again it is not working unless you wait 5 sec between off and on. Is there zny solution ??
 

am.steen

Jul 10, 2023
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any one have a circuit diagram for board in attachment
 

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AnalogKid

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As circuit triggered at power on but when power off and on again it is not working unless you wait 5 sec between off and on. Is there zny solution ??

Add a small signal diode (1N914, 1N4148) across R1 (your schematic), anode to Vcc. This will create a discharge path around the 10K resistor.

ak
 

am.steen

Jul 10, 2023
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Add a small signal diode (1N914, 1N4148) across R1 (your schematic), anode to Vcc. This will create a discharge path around the 10K resistor.

ak
I add the diode but same problem my current schematic in attachment
 

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AnalogKid

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Now that C4 is in place, I think C1 can be reduced 10x.

The discharge problem is that when the switch (SWx?) is opened, there is not a good low impedance path from the Vcc net to GND. A solution is to add another 10K resistor to the circuit, from pin 8 to pin 1 (Vcc to GND). Now C1 will charge up through 10 K and discharge through 20 K.

The input circuit looks strange. Why are there two resistors in parallel with C2, and why is there a 75 ohm resistor across the bridge?

ak
 

am.steen

Jul 10, 2023
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Now that C4 is in place, I think C1 can be reduced 10x.

The discharge problem is that when the switch (SWx?) is opened, there is not a good low impedance path from the Vcc net to GND. A solution is to add another 10K resistor to the circuit, from pin 8 to pin 1 (Vcc to GND). Now C1 will charge up through 10 K and discharge through 20 K.

The input circuit looks strange. Why are there two resistors in parallel with C2, and why is there a 75 ohm resistor across the bridge?

ak
- I add 10k resistor between vcc and gnd same problem except that wait time after power off reduced to 2 sec to trigger 555 on power on. When reduced capacitor c1 wait time increased.
The 2 parrallel resistor with c2 to have 500k resistor with 0.5 ohm and 75 ohm for limiting current
 

crutschow

May 7, 2021
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I add 10k resistor between vcc and gnd same problem except that wait time after power off reduced to 2 sec to trigger 555 on power on.
Then reduce it to 1kΩ or less.
How fast do you want the recovery time to be?
Why does it need to be fast?
 

am.steen

Jul 10, 2023
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Then reduce it to 1kΩ or less.
How fast do you want the recovery time to be?
Why does it need to be fast?
I try 1k and recovery time reduced to 0.5 s. I do not know if this save ??
This will be used for stair lights timer so when some one switch it on another one may swtich it on immediately after it goes off from any floor
 

am.steen

Jul 10, 2023
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In attached board photos the big capacitor c2 is ac filter and the small one c4 and time adj resistor connected to pin 2 so where is the monostable RC ?? Is pin 6,7 free ??
 

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danadak

Feb 19, 2021
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One thinks of the following :

1) There should be a V for which action occurs at that value when > and when >=
What is that V ?

2) Brownouts should not affect logical operation.

3) Pulse generation / time delay should have some level of T and V variation
protection.

4) Pulse generation accuracy issues, is there a spec ?

5) What is considered suitable delay to reset circuit to enable to generate another pulse ?

This all seems to beg a low end processor solution, no large caps to discharge, accurate
pulse generation (onchip clock accuracy of a few % versus), and brownout logic/code handling.


Regards, Dana.
 

AnalogKid

Jun 10, 2015
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In attached board photos the big capacitor c2 is ac filter and the small one c4 and time adj resistor connected to pin 2 so where is the monostable RC ?? Is pin 6,7 free ??

You already have identified the C, and the R is the pot.

This is a double-sided board. There might be connecting traces on the op side of the board beneath the IC.

ak
 

danadak

Feb 19, 2021
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Here is a first pass cut using block language to do the design. Takes 8 pin dip +
interface circuit to measure power V you use to trigger event. As simple as a V
divider off DC if thats what you have.

1689176003998.png


Regards, Dana.
 
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