How to get my osciliscope to see a slow multivibrator circuit? - Page 1
Forum section: electronics / testgear

author: Beamin
author_rank: Super Contributor
timestamp: October 10, 2017, 11:57:08 pm
content: What do I set the time switch to? I have the multi vib. with two different caps that make two leds turn on for 1 sec and 0.5 seconds. I can sort of see the trace if I mess with it but I'm not sure what I'm doing beside randomly spinning the dial until I see something.

author: Octane
author_rank: Regular Contributor
timestamp: October 11, 2017, 12:56:22 am
content: Hi Beamin,I’m not familiar with the Tektronix 2465, but it seems that it is not a storage scope. Those old analog scopes are not very suitable to see very slow signals (like your multivibrator circuit). Theoretically you need to set the timebase to something like half a second or a second, but then you will just see the dot moving from left to right on the screen and it will jump up and down with the signal. So as I said, a purely analog scope won’t do the job.BR,Michael

author: vk6zgo
author_rank: Super Contributor
timestamp: October 11, 2017, 03:56:05 am
content: Most times, to determine if a very slow change in state of voltage is occurring, it is sufficient to leave theOscilloscope "free running" in "auto trigger" setting, with any convenient time/div setting.With the 'scope input DC coupled, this will present a horizontal line across the screen.As the voltage state changes, the line will move up or down the escreen.If you really have to see the actual waveform shape, set time/div so that the sweep takes several times the anticipated time of each cycle, & still in "free -running" mode, wind up the intensity control until you can see the spot tracing out the waveform.Note that this is not a "locked" display, & it may drift across the screen over time.Another good idea is to reduce the lighting levelYou may be able to record this on a digital camera in either the "B" setting, if it has one, or as video.(we used to use a Polaroid camera)Analog storage 'scopes were not all that common in the past, but people got by with the above work-arounds.

author: Beamin
author_rank: Super Contributor
timestamp: October 11, 2017, 10:37:45 am
content: Ok so I changed my circuit to make a much faster pulse. I went from uf caps down to 10's of nf and the LED looks dim; I know when it does that it oscillating very fast.The probe ground is on the neg. of the power supply and the probe is connected between the LED and the transistor. I have got a multivibrator like this to go up to a few hundred kHz; I could see the signal on the water fall of an ADR radio by attaching it to a big coil of wire and placing an antenna on top.Do I set the scope to AC? Sometimes on DC I get a signal. Right now I'm trying to not chase down noise. When I first thought I had found the signal I shut the circuit off and it stayed so that wasn't it. So my goal would be to figure out what frequency my circuit is running at.

author: rstofer
author_rank: Super Contributor
timestamp: October 11, 2017, 01:57:47 pm
content: Quote from: Beamin on October 11, 2017, 10:37:45 amA schematic would be helpful...I'm going to GUESS that you have a resistor between Vcc and the anode of the LED. The cathode of the LED is connected to the collector of the transistor and the emitter is connected to ground.When the transistor is OFF, there probably won't be any voltage at the cathode of the LED because there is no current flow to convince the diode to conduct.So, you are measuring the collector voltage when the transistor is ON (0.2V, probably) and OFF (0V or some other floating value).Probebetween the resistor andthe anode of the LED. Assuming it is wired as I GUESSED.ETA: If you have the resistor between the LED and transistor (LED anode connected to Vcc) then you may have a problem finding anything useful to probe.

author: sprintcarfan
author_rank: Contributor
timestamp: October 12, 2017, 12:22:23 am
content: I'm a noob when it comes to 'scopes, but DC coupling should work. Are you able to see your oscilloscopes calibration signal? If you can adjust everything so that you can see that square wave test signal, you should be able to see your circuit's signal after you've turned the timebase to something slower. Also, the "Beam Find" button might also be helpful in case you've got things too far out of adjustment.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 12, 2017, 02:29:45 am
content: Quote from: rstofer on October 11, 2017, 01:57:47 pmHow will the multivibrator oscillate if one collector is always at or near zero volts?

author: Beamin
author_rank: Super Contributor
timestamp: October 12, 2017, 03:09:57 am
content: Quote from: vk6zgo on October 12, 2017, 02:29:45 amI was wondering that. Sometimes when you turn it on it just lights the leds and you have to "get it going" by turning on/off the power supply. Also this circuit breaks spice because it can't start the vibration.http://www.electronics-tutorials.ws/waveforms/tim20.gif

author: vk6zgo
author_rank: Super Contributor
timestamp: October 12, 2017, 05:14:46 am
content: If my lab/ham shack wasn't chock full of other people's stored furniture & other detritus, I would make one up & look at it with the 'scope!

author: Beamin
author_rank: Super Contributor
timestamp: October 15, 2017, 05:54:01 am
content: Quote from: vk6zgo on October 12, 2017, 05:14:46 amYou could always take it out pile it up and burn it. Then send a picture message of the fire to the owner that says "Hurry up and get your shit".