555 timer oscillating circuit with PNP transistor trouble

Thread Starter

Badbusiness

Joined Nov 30, 2023
14
Hi all, new to the forum.
I’m trying to create a circuit supplied by 12v-15v DC. The goal is to get 0.3v-0.8v oscillating around 1hz-1.5hz. I thought I did it with this circuit but after soldering it together it’s clearly flawed. I understand there is supposed to be a 0.7v difference between the emitter and base and that’s probably a good start to solving my problems but I can’t get anything to work in the real world. Any help is very appreciated. Happy holidays!
46F6AD56-0E30-41E6-B556-6A000278B0D3.png
 

Thread Starter

Badbusiness

Joined Nov 30, 2023
14
Thank you for the fast replies! Glad to see this forum is alive and well.

I made revisions on the EveryCircuit app to show what I actually soldered up and circled in red where the app is showing a waveform in purple, which is where I took my readings (pic1).

I used a 9v battery to trouble shoot and make sure I was getting the proper readings. So on the app, when I reduced the supply voltage from 14.7v to ~ 9.47v (actual battery voltage), the circuit still oscillated around .44v to .25v (pic2).
But when I hooked my 9v battery up, I was getting an oscillating voltage from 1v to 1.3v or so.

I looked into it and figured I had the collector and emitter on the transistor backwards. After switching the transistor leads around, I got 0v on the output and 0v at the “collector”.

Before I go and place yet another order on digikey, I want to make sure I’m not grasping for straws again.

Pic3 shows my protoboard as it sits currently with the transistor switched around as stated above.

Pic4 shows the 555 terminals on the app.

I took electronics in college about 8 years ago but clearly I don’t know what I’m doing.
 

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dl324

Joined Mar 30, 2015
16,918
Pic4 shows the 555 terminals on the app.
1702831384819.png
Could you post the waveform on the threshold pin? I think it should be going to almost 10V, so the peak for the waveform you're showing should be 0.1V lower, not 650mV. The lower voltage should also be higher by almost 5V.

EDIT: Just noticed the resistor you have on the control pin. Will do the calculations when I have time...
 
Last edited:

dl324

Joined Mar 30, 2015
16,918
V(tr) oscillates from .8v-0v which should work but is it possible to raise the minV from 0 to .1-.3v?
Is the 14.7V power supply voltage required? The last LTspice simulation requires changing Vcc to change the voltage swing.

I think you can achieve this by varying the resistor on the control pin and the 500k in series with the cap. It's an interesting application of the control pin and using a current source to charge the capacitor.
 

Thread Starter

Badbusiness

Joined Nov 30, 2023
14
ALSO

I realized the EveryCircuit app isn’t always correct because the first circuit I designed allowed me to get the readings I desired when I had a 1M ohm resistor before pins 8 and 4, which made the VCC 0.8v. When I hooked this up and it didn’t work, I looked into it and you need at least 4.5v for most 555 timers to work.

Beware the EveryCircuit app!
 

dl324

Joined Mar 30, 2015
16,918
Beware the EveryCircuit app!
The same applies to all simulators. They can give unrealistic/incorrect results. It's up to the user to sanity check results. In general, I find them a waste of time.

I've simulated circuits in LTspice that LTspice said wouldn't work. I ended up wasting time tweaking components to get the simulation to run. When I breadboarded the circuits, every variation that I thought should work did. I trust the simulator between my ears more than the various programs.

Have you ever heard that saying about monkeys and typewritters?
 

Thread Starter

Badbusiness

Joined Nov 30, 2023
14
Is the 14.7V power supply voltage required? The last LTspice simulation requires changing Vcc to change the voltage swing.

I think you can achieve this by varying the resistor on the control pin and the 500k in series with the cap. It's an interesting application of the control pin and using a current source to charge the capacitor.
The power supply is a car battery so 12-15v
 

dl324

Joined Mar 30, 2015
16,918
The power supply is a car battery so 12-15v
The circuit suggested in post #9 varies amplitude by varying supply voltage.

I think you'll be better off with your original idea. You can adjust the resistor on the control pin, the resistor in series with the cap, and the charging current.

Since you're going to use this in a car, you should use a voltage regulator and operate at a voltage lower than 12V. Or use a zener diode to make the charging current independent of battery/alternator voltage.
 

Thread Starter

Badbusiness

Joined Nov 30, 2023
14
The circuit suggested in post #9 varies amplitude by varying supply voltage.

I think you'll be better off with your original idea. You can adjust the resistor on the control pin, the resistor in series with the cap, and the charging current.

Since you're going to use this in a car, you should use a voltage regulator and operate at a voltage lower than 12V. Or use a zener diode to make the charging current independent of battery/alternator voltage.
I like the voltage regulator idea. I found a few options on digikey. Back to the drawing board
 
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