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I have been trying to get the following circuit to work with 1.5V AA battery but it does not work, even though it works fine with a 3.3v supply.

I built the multivibrator circuit by discrete components because I haven't been able to find a suitable op-amp or timer IC that works at 1.5v.

circuit

I observed the following in my circuit:

  • At 3.3v supply, the output voltage on 3.3k Ohm load is 4.5v. I did not expect the voltage doubler to actually double the voltage, and I am not using Schottky diodes so I'm okay with with it.

  • At 1.5V supply, though, the output voltage on the same 3.3k Ohm load is 1.4V. That is not really what I expected. I was thinking that I should be able to obtain at least 2V with the circuit but it decreased. I am trying to use this setup to power an ATtiny (works on >1.8V).

I am really puzzled by it because even at 1.5V the oscillator works fine. It even dimly blinks a led when I use a larger capacitor. So why should it not be able to charge two small capacitors.

My question is, what is lacking in my logic? Why are the capacitors not pumping charge at a lower voltage?

Also, if you have any suggestions on how I can make my design more power friendly(maybe a different oscillator design or another voltage doubling topology) please mention them too. Also, please don't suggest charge pump ICs because it is almost impossible for me to obtain them.

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    \$\begingroup\$ Change to Schottky diodes which have a smaller Vf. You may be better using magnetics at a very low voltage ....look up the Joule Thief circuit \$\endgroup\$ – Jack Creasey Aug 18 '18 at 18:24
  • \$\begingroup\$ What's the purpose of Q1, C2, R2 and R3? \$\endgroup\$ – user253751 Sep 14 '18 at 5:51
  • \$\begingroup\$ Q2, C3, R1 and R4 do not form an oscillator. \$\endgroup\$ – user253751 Sep 14 '18 at 5:51
  • \$\begingroup\$ No. Q1, Q2, C1, C2, R1, R2, R3, R4 form an Astable monovibrator. (I used a schematic I found on some website and calculated frequency using a formula.) \$\endgroup\$ – Nirav Sep 15 '18 at 18:21
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With regular diodes, the forward volt drop is generally said to be 0.7 volts and this eats into the peak-to-peak voltage made by your oscillator so, the negative clamping voltage below 1.5 volts (brought about by D1) is more likely to be 0.8 volts and therefore the peak voltage is 1.5 volts higher (due to the oscillator producing 1.5 volts p-p) at 2.3 volts. This is then rectified by another diode (D2) which also "loses" 0.7 volts in the process hence, the final rectified voltage is about 1.6 volts.

Of course, it's more likely that your oscillator is producing 1.3 or 1.4 volts p-p and this means your doubler is running out of steam big time. Try schottky diodes to get maybe 2.5 volts at the output (on very light loads).

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