I don't understand whats going on... - Page 1
Forum section: electronics / beginners

author: Mint.
author_rank: Frequent Contributor
timestamp: June 24, 2012, 06:41:11 am
content: I have got some strange phenomenon going on in my lab and its really off putting me from playing with electronics because none of my circuits work the way they should. I don't understand whenever it is just static electricity or something else. It is probably with me ever since the first time I setup my lab and built a circuit. Back when I was working with transistors is when it got really noticeable. When I construct a circuit and hook an LED through a transistor and power it up while leaving the base not connected to anything the LED stays off. However when I tap the base pin it turns on. I think that the current from me is getting amplified from the transistor and turning the LED on so no problem. But when I connect a circuit up with base connected already and tap various parts of the circuit it begins to act funny. Today I was working on a monostable 555 timer. I constructed the following circuit:It absolutely refuses to work. First of all the LED turns on when I switch the power on, secondly each time I power the circuit on the LED stays longer. Then also if I touch pin 2 with my finger it turns the bloody thing onI don't understand whats going on. I tried changing voltages, breadboards, component values and yet it still doesn't work the way it should. I am getting a bit frustrated with this stupid issue because I've had it for a while nowCan anybody suggest what to do about this?

author: Psi
author_rank: Super Contributor
timestamp: June 24, 2012, 06:47:06 am
content: Just a thought.. did you wire this bit up correctly.The input goes to pin2, it doesn't connect to the vertical wire over it

author: Mint.
author_rank: Frequent Contributor
timestamp: June 24, 2012, 06:48:53 am
content: Quote from: Psi on June 24, 2012, 06:47:06 amYep I got it all correct for that bit. I even reconstructed the circuit 3 times from different schematics just in case.

author: Psi
author_rank: Super Contributor
timestamp: June 24, 2012, 06:51:32 am
content: Check your using the 555 pins with the numbering like this8 7 6 5|-----------------|> ||-----------------|1 2 3 4Also tell us your R and C values

author: Mint.
author_rank: Frequent Contributor
timestamp: June 24, 2012, 06:56:02 am
content: Quote from: Psi on June 24, 2012, 06:51:32 amYup, all good here. Resistor's 100K and the cap is 10uf.

author: Psi
author_rank: Super Contributor
timestamp: June 24, 2012, 07:00:07 am
content: Those values are perfectly fine.If the input is floating you may get some strange results, connect it to VCC with a 10k resistor or something, then you can play around connecting it to ground and see if it triggers the timer.A pic of your actual breadboard/circuit may help us see a problem also.

author: codeboy2k
author_rank: Super Contributor
timestamp: June 24, 2012, 07:07:21 am
content: PSI has a good point, pull up the input to Vcc and trigger it by pulling it down to GND.Also, what are you using for Vcc (battery? power supply) and what is the voltage level? do you have a bypass cap on the breadboard, near the Vcc pin of the 555 ? Most of the time you can get away without a bypass cap for a 555, but if your supply is marginal, it might help.

author: amyk
author_rank: Super Contributor
timestamp: June 24, 2012, 07:47:39 am
content: Quote from: Mint. on June 24, 2012, 06:41:11 amA MOSFET only requires a charge to turn on, but you said base so it's a BJT, which requires a current going through it, and whatever is supplying the current through you is strong enough to provide a potential of ~0.6v at probably several uA.Are there any emitters of strong electromagnetic fields around you?

author: Mint.
author_rank: Frequent Contributor
timestamp: June 24, 2012, 08:13:58 am
content: Quote from: Psi on June 24, 2012, 07:00:07 amHey Psi,I do not understand what you mean floating input. I know what a floating ground is, I think its when there is no ground connected, but floating input?Quote from: codeboy2k on June 24, 2012, 07:07:21 amHey codeboy2k,My VCC is a power supply (Atten 3005S).I have placed an input cap on the circuit plus also added a proper switch, just found it and works a bit better but sometimes it doesn't accept any clicks and if I hold the button and then release it turns on again. It also turns on when I turn the power supply off.Quote from: amyk on June 24, 2012, 07:47:39 amHey amyk,I don't think there are many strong electromagnetic fields around me. Maybe I have lots of electronic devices e.g. digital clock, light, my lab equipment and an amplifier.I have attached a picture of my circuit on the breadboard, if you can't see it clearly please tell me and I will take the photo again.

author: amyk
author_rank: Super Contributor
timestamp: June 24, 2012, 08:25:00 am
content: Quote from: Mint. on June 24, 2012, 08:13:58 amFloating = not connected. Like an aerial, it'll pick up any EM and induce a voltage.

author: Mint.
author_rank: Frequent Contributor
timestamp: June 24, 2012, 08:27:48 am
content: Quote from: amyk on June 24, 2012, 08:25:00 amWhy would anyone want to use that? I don't understandOf course its not floating. It coming from the power supply which is powered by mains.

author: EEVblog
author_rank: Unknown
timestamp: June 24, 2012, 08:36:51 am
content: Quote from: Mint. on June 24, 2012, 08:27:48 amThings can "float" (are not connected) by accident, and it's not uncommon with breadboards where the connection in the breadboard can be bad or intermittent. e.g. if you use too small a wire and your breadboard contacts have been expanded through use.Usually you don't want to float pins, it's very bad news.This is where the art of troubleshooting comes in. If you know a circuit MUST work, and it doesn't, then you have to start measuring things one by one to make sure each pin and signal is working as you expect. The bad news is troubleshooting can be a PITA and take quite some time. The good news if you will learn HEAPS!If that circuit worked first time after you built it, what have you learned?Dave.

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 24, 2012, 08:58:20 am
content: Looks like Mint is still missing the "floating input" point. When your switch is off pin 2 will not be connected to anything, so it will pick up whatever signal there is, probably mains. When you move your hand around the switch this will also influence the input.A simple solution is to put a 10K resistor between pin 2 and VCC. My bet is that your problems will be over.

author: Mint.
author_rank: Frequent Contributor
timestamp: June 24, 2012, 09:18:19 am
content: Quote from: PA0PBZ on June 24, 2012, 08:58:20 amHey PA0PBZ!Thanks for clearing it up for me about floating input. I have placed a 10k resistor between pin 2 and VCC and the circuit stopped working, BUT when I placed the 10k resistor between pin 2 and GND it began to work perfectly!Also thanks to everyone who contributed to this topic!

author: Psi
author_rank: Super Contributor
timestamp: June 24, 2012, 09:43:50 am
content: Quote from: Mint. on June 24, 2012, 09:18:19 amSo you now have your button between pin2 and VCC and a 10k resistor between pin2 and ground ? With it wired like that it will start timing from when you let go the button.If you have it the other way, (button, pin2 <--> ground and 10k, pin2 <---> vcc) it will start timing from when you press the button down.

author: Mint.
author_rank: Frequent Contributor
timestamp: June 24, 2012, 09:54:57 am
content: Quote from: Psi on June 24, 2012, 09:43:50 amAhh of course, I forgot to mention my switch is normally closed.

author: 6502nop
author_rank: Regular Contributor
timestamp: June 24, 2012, 12:49:04 pm
content: Heh. Oops, my bad!Attached is the corrected schematic with the pull-up resistor added. I'll try to update the other thread, too.The original circuit had the trigger input fed from a logic gate, which pulls pin 2 high anyway, so it didn't need the pull-up resistor. I just transferred it as-is.As for the transistors, if you were using your original Darlingtons, they have a very large Hfe (gain) by nature, so when you leave the base open, they have nothing to do but amplify anything and everything - even parasitic voltages. Once that voltage hits the base turn-on threshold, it turns on. Once properly biased, it'll only do that when the set conditions are met. I remember a schoolmate of mine once brought in a simple UJT (remember those?) circuit. That thing was so sensitive, just waving your fingernearthe base turned on the attached LED. It made a great proximity sensor.Lastly, about that switch: it looks like an Omron variety, which should have two marked terminals: NO = Normally Open, and NC = Normally Closed. Tie the COM(mon) to GND, and the NO to pin 2 with the pull-up resistor, as the new schematic shows.nopIt's always somethin'...