555 Monostable circuit C1 not discharging - Page 1
Forum section: electronics / beginners

author: SkipMorrow
author_rank: Contributor
timestamp: July 31, 2017, 01:24:09 pm
content: I built my 555 monostable circuit on my breadboard and it worked perfectly. I even scoped the trigger and watched C1 charge and discharge and I had a nice LED to show the output (thanks for the help in my other post guys!). I moved it to my prototype board and soldered everything in place, with the exception of the LED because now I will need it to control a relay instead. But before I went any further, I thought I would check things out. It's a good thing I did because now it seems that C1 isn't discharging when the time delay is up. I remember when it was on the breadboard that C1 would slowly start charging after triggering and when it gets to the right point (2/3rd VCC???), the time delay would end and C1 would discharge immediately. But now I don't see C1 discharging at the end of the time delay. It just seems to charge and charge and charge.I do see the pushbutton correctly in the trace on the scope, so I don't think that is the problem. From the symptoms, it would almost seem that the circuit thinks it is in a constant trigger.I'm at work now, so I don't have the circuit in front of me. And I didn't think to just test pin 3 (output).But I was wondering, could it be that the C1 won't discharge unless something is connected to the output? Right now I have nothing connected to pin 3. Or is it more likely that I gooned something up when soldering everything in place?

author: Zero999
author_rank: Super Contributor
timestamp: July 31, 2017, 02:53:43 pm
content: Is pin 7, the discharge connected properly?Is there a 10nF capacitor between pin 5 and 0V?Is the power supply properly decoupled?Is there a diode in reverse parallel with the relay coil, to protect the 555's output stage from the back EMF generated when the coil turns off?

author: floobydust
author_rank: Super Contributor
timestamp: July 31, 2017, 05:55:55 pm
content: Don't forget pin 4 (/RESET) tied to Vcc

author: SkipMorrow
author_rank: Contributor
timestamp: July 31, 2017, 06:04:33 pm
content: When I get home I will check all of those. Thanks!!!

author: SkipMorrow
author_rank: Contributor
timestamp: July 31, 2017, 08:40:25 pm
content: Ha! I'll bet you guys didn't know it makes a difference if you tie pins 4&8 together (the way the "book" says to do it), or do it my way, which it to connect pins 4&5 together. Groan... (that's was sarcasm). Temporary loss of pin counting ability.... Floobydust wins 100 internet points for correctly guessing the problem.Anyway, soldered it up correctly this time and it works perfectly.So, tell me about that "reverse parallel diode" that I will need for the relay coil.... I wasn't expecting that. I am using a 1.5V panasonic relayhttp://www.mouser.com/ds/2/315/discon2011e_tx-1075957.pdfhttps://www.amazon.com/gp/product/B00DKYKXO6

author: Zero999
author_rank: Super Contributor
timestamp: July 31, 2017, 09:14:47 pm
content: Why use a 1.5V relay? An ordinary 555 timer won't work from just 1.5V. From memory, it needs at least 4.5V to work properly but check the data sheet.Searching for relay reverse parallel diode, finds the answer on the first result.https://www.google.co.uk/search?q=relay+reverse+parllel+diode&ie=utf-8&oe=utf-8&client=firefox-b&gfe_rd=cr&ei=Tp1_Wd_LKOnR8gfi-r9g

author: SkipMorrow
author_rank: Contributor
timestamp: July 31, 2017, 10:04:52 pm
content: The project I am working on are some LED locker lights for my daughter. I want a the lights to stay on for no more than five minutes. Last year we tried a micro-switch ala-refrigerator style, but the switch kept moving and kept on failing (I can't use anything terribly sticky like double-sided tape or glue or screws). So I chose the CMOS 555 for its low standby current. The 555 will turn on a relay to turn on the arduino. So I chose a nice small relay. It's only a 1.5V relay, but I was planning on sticking a 36 ohm resistor in series with the coil (the coil is 16 ohms) I am using three D-Cell batteries for power. I don't think the 555 can power the arduino itself, so I am using a relay.Thanks for the help with the reverse parallel diode.