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Project: Charge Pump Using Astable Multivibrator

LCD text module that is available in most of the country is 5 V type. Recently, many microcontrollers are able to operate below 5 V, and some are limited to only 3.3 V (3.6 V max). If the supply voltage is limited to only 3.3 V, there will be problem interfacing with the LCD module. Usually, boost regulator in IC package is used. Without using any IC, 5 V can be obtained from 3.3 V too.

Schematic



Q1, Q2, C1, C2, R1, R2, R3, and R4 form an astable multivibrator to provide oscillation for switching the next stage.
Q4 is turned on via R4 and R6 when Q1 is off. Then C3 is charged at
3.3 V – 0.7 V (Vdrop) = 2.6 V.
When Q1 is turned on, Q3 is turned on too via R7. The voltage appear at the collectors of Q3 and Q4 is approximately 3.0 V (0.3 V Vce drop). With respect to ground, the voltage at the junction of D1, D2, and C3 is pumped up to
2.6 V + 3.0 V = 5.6 V
D1 plays an important role here to avoid the current flowing into the supply site.
This is for one complete cycle, charging and pumping.
While the next cycle is charging up, D2 and C5 are used to store the current for the previous cycle. The voltage appears at the output is
5.6 V – 0.7 V (Vdrop) = 4.9 V
Rload is used as the load. 4.9 V/1.5 k = 3.27 mA which is more than enough for the LCD

Result



Bill of materials



PCB layout



Actual picture





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