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Inductor-based astable 555 timer circuit

The 555 multivibrator circuit has develop into a default alternative of many hobbyists and even professionals up to now 50 years. Its pair of equations for frequency and obligation cycle simply offers capacitor-resistor combos which fulfill the required output. However there might be conditions when utilizing inductor-resistor values as a substitute could be a greater resolution. An instance is a high-temperature atmosphere which may shorten the helpful lifetime of electrolytic capacitors. In that case, the circuit in Determine 1 is price contemplating.

Determine 1 A 555 multivibrator circuit that makes use of inductor-resistor values.

At start-up, the voltage at pin 2 (VRB) is zero and the output (VOUT) at pin 3 is excessive.  Throughout this time the LED is on and the discharge transistor (pin 7) is open. The inductor present will increase exponentially till VRB reaches 2VCC/3. That is sensed by the brink terminal (pin 6) which causes VOUT to go low. The discharge transistor activates and the LED turns off.

Pin 7-to-ground successfully gives a low-resistance path (RON) which causes the inductor present and VRB to regularly lower in the direction of zero.  When pin 2 senses that VRB has fallen under VCC/3, VOUT goes excessive once more, the LED activates, and pin 7 reverts to its open state.

To calculate the following values of excessive time (TH) and low time (TL) of VOUT, we will use Equation 1 which comes from the step-response evaluation of an R-L circuit.  It offers the period for charging (or discharging) present to vary from an preliminary to a remaining worth:

                          (Equation 1)

The symbols in Equation 1 are outlined in Desk 1.

Desk 1 Formulation to foretell the timing traits.

As well as, RS is the inductor’s DC resistance which isn’t negligible when the standard issue (Q) is low. There may be additionally RON = 59.135 / VCC0.8101 in response to Rogers [Reference 1].  Making use of these within the efficient circuit throughout every interval, the symbols take up specific values as proven within the columns for TH and TL of Desk 1. It’s useful to do all of the calculations on a spreadsheet, particularly after we need to learn how delicate the output is to the tolerances of the parts.

To check these concepts, I picked the parts listed in Desk 2 and used an LCR meter to measure their precise traits (Determine 2). I plugged the values into the spreadsheet to foretell TH, TL, and different output traits.  The outcomes of those calculations are listed in Desk 3.

Desk 2 Elements for the experimental circuit.  

Determine 2 RS of the inductor.

Desk 3 Predicted versus measured values.

I used the ADALM2000 module (from Analog Units) to energy the experimental set-up (Determine 3a) and get the waveforms (Determine 3b) from pins 2 and three of the timer IC. The measurements present that Equation 1, in tandem with these of Desk 2, carefully fashions the habits of the proposed circuit. There have been droops and spikes famous for VRB throughout the transitions at pin 7, principally as a result of lengthy energy strains. These have been mitigated by the pair of capacitors positioned very near pin 8. 

Determine 3a The experimental set-up.

Determine 3b Waveforms of VRB and VOUT, and measurements for VOUT.


  1. Phil Rogers. Design low-duty-cycle timer circuits. EDN (Aug. 22, 2002) pp.98-100.

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