555 Problem - Page 1
Forum section: electronics / beginners

author: codingwithethanol
author_rank: Regular Contributor
timestamp: August 07, 2019, 05:01:52 pm
content: I designed a small circuit for a friend of mine, and it was working fine up until a certain point and now I'm seeing some odd behavior. Basically I just have two 555 timers, the output of one controls the other via the control pin. The problem is that when i turn it on it will only work for one cycle, and then give up. I probed around and discovered that the capacitor isn't discharging through the timer, even though its at 2/3 Vcc. I have no clue what the issue is, as the circuit worked flawlessly when it was breadboarded. Any help given would be very much appreciated.

author: mikerj
author_rank: Super Contributor
timestamp: August 07, 2019, 05:40:52 pm
content: You don't have any supply decoupling capacitors.

author: codingwithethanol
author_rank: Regular Contributor
timestamp: August 07, 2019, 06:17:03 pm
content: I didn't think it'd be an issue, but ill add some. I'm getting stable 5V, this seems to be an issue with the 555

author: codingwithethanol
author_rank: Regular Contributor
timestamp: August 07, 2019, 07:04:59 pm
content: I added the caps as per the suggestion but the situation is the same

author: floobydust
author_rank: Super Contributor
timestamp: August 07, 2019, 07:22:57 pm
content: I see a few issues with the circuit.I don't see the approach working because the second 555 timer control voltage pin 5 is driven hard to near 0V or 5V so it can't oscillate. If your 5V rail was really noisy, which it should be with no decoupling cap or a weak battery, the circuit might have fluked and worked due to it's own noisy power.If you want timer 1 to gate timer 2, try connect timer 1 pin3 to timer 2 pin 4 instead of pin 5.If the pot across pin 7-8 is dialed to zero ohms, it can damage that 555 due to overcurrent on the discharge pin. You need a small series resistor to prevent that like, maybe 220R.I would add a 10-100uF cap across the 5V rail because a heavy speaker load does make power noisy. You don't even need the 5V reg and can run the 555's direct off 9V - if you have good bulk capacitance across the 9V. It would be pretty loud though and have to add a resistor.The loudspeaker out diode seems not the greatest, it does not limit the current into the loudspeaker enough and the 555 can only push the speaker cone out and not in. Instead of the diode, use a resistor 47-220R to drive the speaker.

author: codingwithethanol
author_rank: Regular Contributor
timestamp: August 07, 2019, 07:51:34 pm
content: @floobydust"I see a few issues with the circuit.I don't see the approach working because the second 555 timer control voltage pin 5 is driven hard to near 0V or 5V so it can't oscillate. If your 5V rail was really noisy, which it should be with no decoupling cap or a weak battery, the circuit might have fluked and worked due to it's own noisy power."Interesting, I added the decoupling caps as per the other suggestion, so that should at least be one less thing to worry about.If you want timer 1 to gate timer 2, try connect timer 1 pin3 to timer 2 pin 4 instead of pin 5."I have a Vcc bodge wire between the resets, ill desolder it and make a test connection.EDIT: Actually the problem still is that timer 1 refuses to work, so I think ill save that for later."If the pot across pin 7-8 is dialed to zero ohms, it can damage that 555 due to overcurrent on the discharge pin. You need a small series resistor to prevent that like, maybe 220R."Eeks! I thought the first timer might have been a lemon, so I swapped it with a fresh one. Could I possible have damaged both?"I would add a 10-100uF cap across the 5V rail because a heavy speaker load does make power noisy. You don't even need the 5V reg and can run the 555's direct off 9V - if you have good bulk capacitance across the 9V. It would be pretty loud though and have to add a resistor."I actually only added the regulator because I had only tested it on a breadboard at five volts. I actually checked the 555 datasheet after I added it and discovered it could operate from 4.5-14 something volts, but i didnt feel like desoldering the regulator assembly so I said fuck it and kept going."The loudspeaker out diode seems not the greatest, it does not limit the current into the loudspeaker enough and the 555 can only push the speaker cone out and not in. Instead of the diode, use a resistor 47-220R to drive the speaker."I actually added the diode as an afterthought, as I remember from an earlier experiment that speakers can produce back emf, and I thought that it might damage the timer. Also I have a 3 pin header with a jumper, which routes the output through either just the diode, or the diode in series with a 10 ohm resistor as a jank volume control solution. It's actually very low even with just the diode, far lower than when I had prototyped it.

author: floobydust
author_rank: Super Contributor
timestamp: August 07, 2019, 08:05:48 pm
content: yes a loudspeaker has inductance and makes a -ve back EMF that can damage IC's. But the diode must connectacrossthe loudspeaker, not in series with it. I would move the diode (cathode to spkr+, anode to GND) and have a series resistor to limit current into the speaker to under the555's rated 100mA.10R is too low, about 40R is the lowest, so I suggested 47R.

author: codingwithethanol
author_rank: Regular Contributor
timestamp: August 07, 2019, 08:51:33 pm
content: Ok, I gotta find another diode and then ill put it directly on the speaker. Also, if you have any experience with audio hardware, how would I get the speaker to line level? For example, so you could connect it to a speaker via 3.5mm jack and not toast it? Anyways, thank you for the help

author: Audioguru again
author_rank: Regular Contributor
timestamp: August 07, 2019, 10:56:18 pm
content: A 555 is not an audio power amplifier. Your 8 ohm speaker overloaded it. It worked better before but now its loudness "is far lower than when I prototyped it" so the 555 is damaged.