Hi,
I’m working on a project designing a modular synthesizer. My envelope generator includes a 555 timer to control the timing of 4 parameters. Attack, decay, sustain and release. I’m using a +/-12V power supply to power the circuit and a 5v pulse to trigger the timer. Can someone offer advice as to why my lm555n timer is overheating almost as soon as I turn the power supply on? Any advice is appreciated. this is not my schematic, I found it online with little explanation. I’m still an engineering student!
The NE555 maximum voltage is +18 Volts, so if you feed it +12 V and -12 V, you exceed its limitations. But that is probably not what you are doing. The op-amp is connected to +12 and -12, the 555 is connected to +12 and GND, so that looks good (in the schematic). To be on the safe side: you are using a DC power supply? Have you measured for shorts between the pins of the 555?
Why is there a connection between pin 4 and GND? That pin is normally connected to +
What’s more, the schematic and the connections to C1 and pin 4 and C3 and the Release 1M do not make much sense if the connection to GND is made. Is this a redraw of a schematic you found? Please check these parts.
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The eurorack power connector seems to be connected the wrong way.
1-2 should be -12V
9-10 should be +12V
You better check the polarity of the power supply and the polarity of the voltages on the supply pins of the ICs.
Hi, I just saw your reply. Yes, this is a schematic I found online and decided to try it out. I realized i sent the manufacturers the schematic with pin 4 accidentally grounded. I have updated my circuit using air wires to resolve that issue and the overheating has stopped. However, the reading on my oscilloscope does not show the ADSR shape I expected nor do the pots seem to have any effect on the output. Any more general advice for PCB help? 
One thing you need to know is whether the 555 is still OK.
If you apply a signal to the circuit does the 555 then receive a trigger pulse on pin 2? If it does and the rest of the circuitry is OK, then the 555 should produce a change on pin 3.
If you think the 555 is OK but it does not get a pulse from the input circuit, then you could try to trigger the 555 yourself by shorting Q3: for a brief moment connect the collector to the emitter. You can do this without fear, you are merely doing what the transistor is there for in the circuit, in other words, the transistor is configured to work as a switch and you are taking over that function. If you then get an adsr, you need to debug the circuit preceding the 555 and Q3.
You could try shorting Q2 for a brief moment and see whether that results in a pulse that opens up Q3. You can go back towards the input even more and do the same with Q1.
Let us know how you fare.
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Hi, so I decided to breadboard my circuit to trouble shoot and I’m noticing that my 555 timer is outputting a negative voltage spike. I’m unsure what this means.
This is what output 3 reads when I manually apply a 5v pulse using a function generator.
Scope in AC mode?
ㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤ
It is in DC mode. Do i want it in AC?
The 555 output swings between 0 and a bit under VCC, so you want DC coupling, otherwise the scope will try to get rid of the DC average which will mess with the signal – you get negative spikes for falling edges, positive spikes for raising edges. Your screenshot looks like there’s a capacitor on the way, so if it’s not in the circuit (not seeing anything in the schematics) and it’s not the scope it’s somewhere in between
Man… I cannot figure out what is causing this problem. For a second I got what looked like ADSR for a couple minutes but it’s gone again. I might be overloading my circuit with too much current but im following the schematic I found online. I dont know why it wouldnt work.
it can also just be a schematic with errors as there are plenty of them on the net.
So clicking around a bit, your schematics looks like René Schmitz’ second version here:
https://www.schmitzbits.de/adsr.html
except that RESET (and the output from Q2) is pulled to ground by the connection here:
This forces OUT low, and turns on the discharge transistor, which pulls DIS to ground. Not sure if this can cause overheating, but it definitely messes with the operation of the circuit.
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Hi,
I already corrected that problem by breadboarding but my output is still incorrect. No overheating anymore though. Thank you!
As I understand it you’re using a PCB made from the schematic you posted, with pin 4 and the release pot disconnected from ground (and connected to each other). As @fredrik points out this is Schmitz’s circuit, which should be good, and no other errors are evident in comparing to your schematic. So if your board isn’t working, there’s an error in the assembly. If you’ve gone through all these steps:
https://lookmumnocomputer.discourse.group/t/general-advice-for-troubleshooting-a-module/2049/2
then the next thing would be to post pictures of both sides of your assembled PCB to get other eyes on it looking for problems. Or, I assume you have 4 more copies of the PCB, you could try assembling another one and see if it behaves better.
Have you switched out the 555 for another one? In case the first got damaged when it overheated?
