Posted on Apr 27, 2012

When configured as a free-running multivibrator, a 555 timer provides no more than a 50% duty cycle. By adding two transistors, however, you can obtain a variable 5 to 95% duty cycle without changing the sum of the on and off times. When VouT decreases, Ql is on and Q2 is off, disconnecting V + while timing capacitor C2 discharges into pin 7 of the timer. When VoUT increases, Q2 reconnects V + for recharging C2.

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Adjusting linear trimming potentiometer R3 to increase the charging resistance increases the on time, but decreases the off time by the same amount by lowering the discharge resistance (the converse is also true). As a result, the sum of the on and off times remains constant. R2 protects Q2 and the timer against high-charge/discharge currents

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