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Help with 555 latch circuit

cagz

Feb 13, 2021
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Hello,
New to the forum and my first post !

I am trying to build the circuit in the last picture. Aim is to turn on and off the led in a alternating way on each press of the button.
Circuit seems to be working on this video:
and also when I replicated it on tinkercad. However, for some reason no success on the breadboard. I must be doing something silly but no success after a lengthy troubleshooting.

The problem is that, on the first press of the button, led goes on, however subsequent presses does not turn it off. All R/C values are triple checked. I've also observed the voltage on pins 2/6. As expected voltage stays stable at 2.5V (Vcc=5V) when there is no button action. First press actually creates a spike down to 0V, as expected again, and led turns on. However, when I press the button again, next voltage spike only goes upto 3.01V (not enough to hit the threshold of 2/3Vcc (3.3V) to turn to led off again.

In a desperate attempt I tried different C values, but peak always goes up to 3V. Also tried to change R1 to raise Vstable towards 3.1V it didn't help (perhaps was a bad idea).

1 - What would you recommend as the next step of troubleshooting ?
2 - If you are patient enough, can you see an issue with my breadboard setup ?
3 - Any other ideas to fix it ?

Many thanks,

Charles
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Image 13-02-2021 at 17.28.jpg
 
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AnalogKid

Jun 10, 2015
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The original 555 device (NE555, LM555, MC555, etc.) is a bipolar device. If you read the datasheet, you will see that the output does not go all the way up to Vs (should be called Vcc) under any conditions, and especially when it is trying to source 11 mA into a load. The CMOS 555 (LMC555) has a different output stage that swings much closer to its positive rail. However, that device can source only 10 mA.

If you increase the power supply voltage to 9 V or 12 V, the circuit will work.

ak
 

cagz

Feb 13, 2021
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Feb 13, 2021
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@AnalogKid thanks a lot, indeed I missed this point.
Seems like I still have some other problem. I increased voltage to 9V, this increased pin2/6 voltage to 4.2V, and it only jumps up to 4.5V when I push the button. Still nowhere near V2/3 of 6V. 555 I use is ICM7555IPAZ, but I didn't see anything awkward on the data sheet. I must be doing something else wrong, any more ideas ?
 

Alec_t

Jul 7, 2015
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Try reducing the 100k to 4.7k. You may then need to increase the 1uF to, say, 22uF.
 

AnalogKid

Jun 10, 2015
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100K and 1 uF is a time constant of only 0.1 s. This will not register on a meter. The intent of the circuit is that there is an intentional voltage spike on the 555 inputs caused by the capacitor when it is fully charged up by the 555 high output. But the 5 K input impedance caused by the two 10K resistors is too low. They reduce the circuit time constant fo approx 5 ms, and are discharging the capacitor voltage before it crosses the Threshold input transition voltage. Remember that the bipolar 555 is not the fastest chip on the block.

Increase R1 and R2 to 100K.

ak
 

Alec_t

Jul 7, 2015
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If R1 and R2 are both made 100k that will certainly help, but the existing 100k should be reduced to ensure >2/3 of the supply voltage and < 1/3 of the supply voltage is reached when the button is pressed.
 

cagz

Feb 13, 2021
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Thanks for all the suggestions. I tried different values of resistors / capacitor but keep getting a too small of a spike to pass 555's threshold. What made it work in the end was when I reduced Vcc to 3.3V. It was a hunch, an I am still not very clear why that helped. Having pin 2/6 around 1.6V, the spike was enough for it to pass 2.2V and turn the led off.
Thanks again.
 

Audioguru

Sep 24, 2016
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Your ICM7555 is a Cmos 555 that is completely different to an ordinary NE555 or LM555. It has a low output current that is loaded down with the LED current. It will probably work if you add an NPN emitter-follower to its output to drive the LED.
 

Bluejets

Oct 5, 2014
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Thanks for all the suggestions. I tried different values of resistors / capacitor but keep getting a too small of a spike to pass 555's threshold. What made it work in the end was when I reduced Vcc to 3.3V. It was a hunch, an I am still not very clear why that helped. Having pin 2/6 around 1.6V, the spike was enough for it to pass 2.2V and turn the led off.
Thanks again.

Just wondered if you are allowing enough time for the cap to charge or discharge..?
Problems on breadboards usually point to bad connections somewhere.
 
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