Pls help beginner debug simple astable multivibrator circuit

Thread Starter

cwgstreet

Joined Aug 14, 2016
7
I'm learning electronics and trying to build a simple astable multivibrator circuit as an oscillator which will eventually drive a couple of small 3V DC motors for a slithering snake toy. I've also used this project as an opportunity to start learning EAGLE and also to etch my first circuit board for fun.

To test the oscillator, I've just hooked up the motor connectors to a couple of LEDs / resistors on a breadboard (instead of the 3V DC motors). I expected to see the LEDs flash back and forth.

Unfortunately, the built circuit doesn't work as expected - when I first power it, I get what looks to be brief flickering of the two LEDs and then they both stay lit up. I'm at wits end trying to debug this. I can't find any issue with my board or components. I've even played around with simulating it in LTSpice to see if I can recreate the behaviour I'm observing but the simulation has it oscillating as expected.
snake-ltspice-output.jpeg snake-eagle board.jpeg
Grateful if anyone can see an obvious mistake I am making or perhaps suggest an effective troubleshooting process. Below is the LTSPICE circuit / simulation & the EAGLE board that I built.

Thanks!
 

LesJones

Joined Jan 8, 2017
3,296
2.4 volts is not enough. The voltage drop across a red LED is 1.8 volts and the forward volts drop of the 1N4148 is 0.6 volts so there is nothing left at the collectors.

Les.
 

Thread Starter

cwgstreet

Joined Aug 14, 2016
7
C1, R3 are wired wrong at Q2 base PCB.
Pls elaborate as I don't see it. R3 (and R4) above are just 0-ohm resistor (jumper on my board). Eagle schematic / board are tied together in the software and I thought my schematic is same as what I simulated. I've checked the as-built board against my schematic and haven't found any errors? Below is the board / schematic side-by-side. Thanks!

snake-eagle-side-by-sde.jpeg
 

Thread Starter

cwgstreet

Joined Aug 14, 2016
7
2.4 volts is not enough. The voltage drop across a red LED is 1.8 volts and the forward volts drop of the 1N4148 is 0.6 volts so there is nothing left at the collectors.

Les.
Thanks, Les. This makes sense to me. The original idea used a 3.7V LiPo but we wanted to move away from LiPo's and just use a couple of rechargeable AA batteries. I'll test out a higher voltage supply.

One question - why does the circuit simulate okay in LTSPICE with 2.4V? Pls see original pic of simulation output. Cheers!
 

crutschow

Joined Mar 14, 2008
27,463
why does the circuit simulate okay in LTSPICE with 2.4V? Pls see original pic of simulation output.
The actual LEDs likely have a higher forward drop then the ones in the simulation.
Use some actual LED models in the simulation, not the default model.
 

LesJones

Joined Jan 8, 2017
3,296
A resistor between the collector of each transistor and the +2.4 volt rail MAY make it work. I would suggest 470 ohms as a starting value. What is the purpose of the 1N4148 diodes ? I can't see that they serve any purpose.

Les.
 

Thread Starter

cwgstreet

Joined Aug 14, 2016
7
?..What is the purpose of the 1N4148 diodes ? I can't see that they serve any purpose.

Les.
I've wondered the same thing - perhaps to make the oscillation rise sharper? I'm new to electronics and used this as one of my learning projects - point was to build a few boards that my wife could use in in Makerspace toy project in a school library.

I got the idea from this video:


Schematic (at 3:00) shows the signal diodes.

I haven't had the chance yet to try a higher voltage supply.

Thanks everyone for your feedback & comments.
 

Thread Starter

cwgstreet

Joined Aug 14, 2016
7
Just an observation; The zero ohm resistors/links could have been avoided by routing the PCB differently.
You are clearly better at routing than I am!! Do you want to explain how for my learning? This is the first board I ever routed and I literally spent hours trying to get the layout without jumpers and finally gave up. Perhaps I'll improve over time...
 

blocco a spirale

Joined Jun 18, 2008
1,546
You are clearly better at routing than I am!! Do you want to explain how for my learning? This is the first board I ever routed and I literally spent hours trying to get the layout without jumpers and finally gave up. Perhaps I'll improve over time...
I would also change the component layout but there are a few ways it can be done even with the existing arrangement.
PCB 02.png
 
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Thread Starter

cwgstreet

Joined Aug 14, 2016
7
I would also change the component layout but there are a few ways it can be done even with the existing arrangement.
View attachment 118916
Excellent! Thanks so much for illustrating it - very helpful as I learn.

I'm also wondering if part of the issue are the relaxed design rules I set in Eagle (0.8 mm trace width / 0.8mm spacing - over 30 mils!) to make it easier to etch my first board. I'll tightened these up as I get better with toner transfer etching.
 
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Thread Starter

cwgstreet

Joined Aug 14, 2016
7
2.4 volts is not enough. The voltage drop across a red LED is 1.8 volts and the forward volts drop of the 1N4148 is 0.6 volts so there is nothing left at the collectors.

A resistor between the collector of each transistor and the +2.4 volt rail MAY make it work. I would suggest 470 ohms as a starting value. What is the purpose of the 1N4148 diodes ? I can't see that they serve any purpose.
Les.
Tested circuit with larger 5V supply and it oscillates correctly, just as you noted, Les. Animated GIF below.

astable_multivibrator.gif

I also tried your suggested 470ohm resistor and this works as well. Can you pls help me understand why this works? It seems to be pulling the collector up to Vcc but not sure why this starts the oscillation when it would not work before at the low 2.4V supply.

I'm going to have to play with this a bit to get a lightweight power supply and functional oscillator. I'll make this as solved. Thanks everyone and especially Les.

Carl
 

LesJones

Joined Jan 8, 2017
3,296
Using the circuit in post #1 with Q2 on the left. When Q2 is in the off state the positive end of C1 needs to be pulled up to the positive rail so it will charge up. With just the LED and the diode the collector of Q2 can not get anywhere near the positive rail voltage due to the voltage drop across these two compnents. Putting the resistor from the collector to the positive rail allows the collector to go up to the positive rail voltage when Q2 is turned off thus charging C1. At some point in time when C2 has been discharged by R3 Q2 will switch on. This will pull the positive end of C1 to near ground and as C1 is now charged its negative end will become about 2 volts negative of ground turning Q1 off. C1 will now be discharged via R4. When the negative end of C1 reaches about + 0.6 volts Q1 will conduct which will turn Q2 off via C2. The process then repeats.

Les.
 
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