Problem with Astable Multivibrator Driving Circuit

Thread Starter

FranciscoB

Joined Feb 8, 2014
66
I breadboarded an Astable Multivibrator Driving Circuit which makes two LED blink at the rate I want but when I use an LED stick it looks dim, not fully bright.

I started with this theory:

https://www.electronics-tutorials.ws/waveforms/astable.html

Then put together this circuit (Falstad):

https://tinyurl.com/yh48yt8n (LED blink at around 1Hz, transistor are 2N3904 and I have tried some BC547 and 2N2222 with the sam result)

I would like to replace the red LEDs with an LED stick. These work at 5V and seem to pull close to 350mA.

When I replace the red LED+resistor with the LED stick, it blinks at the correct rate but it doesnt appear to be fully ON.

Here is a video:
(in this video they are blinking faster because I was testing different resistors)

(I have some AO3400 N-Mosfets and AO3401 P-Mosfets here I that's the way)

Thank you in advance.
 

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ericgibbs

Joined Jan 29, 2010
15,378
I would like to replace the red LEDs with an LED stick. These work at 5V and seem to pull close to 350mA.
Hi FB,
That circuit is not suitable for driving that LED stick.

E

Update:
You need an addition like this from that link.
Choose a 2N2222 for the low side driver.
E
EG 1302.gif
 

Thread Starter

FranciscoB

Joined Feb 8, 2014
66
The circuit I posted above and that I am working on already includes TR3. At the moment it's a 2N3904 but I already tried with a BC547 and 2N2222 as per my initial post.
 

dl324

Joined Mar 30, 2015
14,353
At the moment it's a 2N3904 but I already tried with a BC547 and 2N2222 as per my initial post.
Different transistors will have no material effect on the circuit. BC547 has a higher beta and 2N2222 has a higher maximum current; neither factor is relevant to your circuit.

Are the LEDs you're using bright enough for you at 5mA?
 

Thread Starter

FranciscoB

Joined Feb 8, 2014
66
Thank you for your time, eric. That is the circuit I am using now.

Dennis, the red LEDs seem fully On, as I have tested with another red LED connected to 5V by their side.
 

ericgibbs

Joined Jan 29, 2010
15,378
Hi FB,
If that's the circuit you are using now, why did you post the earlier one.??

Post your new complete circuit, so we can see the problem.
E
 

LesJones

Joined Jan 8, 2017
3,702
Even if the three LEDs are red ones (Which have the lowest forward voltage which is about 1.8 volts.) you would need 5.4 volts plus a bit more so that the current limiting resistor can regulate the current. As your supply is only 5 volts the current through the LEDs will be very low. Use just 2 LEDs in series and adjust the value of the series resistance to give required LED current.

Les.
 

sghioto

Joined Dec 31, 2017
3,071
I would replace Q3 with the mosfet you have on hand the AO3400. The darlington arrangement of Q2 and Q3 will not allow the collector of Q3 to saturate fully. If the LED strip is designed for 5 volts then you don't need a series resistor.
1637786024071.png
 

Thread Starter

FranciscoB

Joined Feb 8, 2014
66
Have you tried that circuit using the LED strips?
Im currently using 1W 3.6VLED beads but I tried with those USB LED sticks hand they seem fully bright now. I think I might have damaged the AO3400 n-mosfets during the experimentation phase, hence the weir behaviour.

If you used AO3400, you could remove the two BJTs and 2 resistors.
Found some theory here: https://www.homofaciens.de/technics-base-circuits-multivibrator_en.htm
Here is the circuit simulation: https://tinyurl.com/y66bqqwf
 

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Thread Starter

FranciscoB

Joined Feb 8, 2014
66
Regarding the project shown in the photos above, even tough it worked without a problem for almost 10 hours on the breadboard, the PCB version showed some malfunction in the field. The 1W LEDs would start to blink progressively faster until one of them would turn OFF. I replaced these 1W LED (and their respective long cable) on the PCB with a couple of small red LEDs with 1K current resistors. I plugged this version to the same power supply (a 5V powerbank), and it's behaving normally. I wonder if the current pulled by the 1W LEDs or the long cables were causing trouble.

Anyway, I moved on to using an NE555, a p-mosfet and an n-mosfet to simplify the project and make it less prone bad soldering or other mistakes from my part: https://tinyurl.com/y65ehc5v (Falstad simulation)
 

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Last edited:

LowQCab

Joined Nov 6, 2012
1,931
If You would like to have an exact ~50% Duty-Cycle,
there is a better 555 circuit configuration ........
( use Pin-3 to drive your MOSFETs )
( Pin-2 creates a Triangle-Wave which may give You some interesting effects )
.
.
.
7555 Triangle Oscillator FLAT .png
 
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