---
title: "Astable multivibrator circuit (555 or BJT)"
subject: "Electronics Devices and Circuit"
chapter: 5
key: "semester_3/electronics_devices_and_circuit:amv-circuit-draw"
source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/amv-circuit-draw
site: BCT Engineering
---

# Astable multivibrator circuit (555 or BJT)

Electronics Devices and Circuit · Chapter 5: Signal Generator and Waveform-Shaping Circuits

- Asked in: 2070 Chaitra, 4 marks
- Video: [Astable Multivibrator circuit explained in detail | Using BJT](https://www.youtube.com/watch?v=ROJNpp5GI-Q) — HK CircuitEdu
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/amv-circuit-draw
- Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-5

## Question

Draw AMV circuit using IC 555 or BJT.

## Short answer

Using a 555 timer, RA from VCC to pin 7, RB between pin 7 and pins 2/6, capacitor from those pins to ground. Output frequency f = 1.44/[(RA+2RB)C]. A BJT astable uses cross-coupled RC pairs with frequency f = 1/(0.693(R1C1+R2C2)).

## Answer

#### Astable multivibrator using IC 555
```text
              +VCC
               |
       +-------+---------+
       |       |         |
     [RA]     (8) VCC   (4) RESET
       |
       +----------------(7) DISCHARGE
       |
     [RB]
       |
       +-------+--------(6) THRESHOLD
       |       |
      [C]      +--------(2) TRIGGER
       |
      GND               (3) OUTPUT ---> square wave
                        (5) CONTROL --[0.01 µF]-- GND
                        (1) GND
```

- Pin 8 (VCC) and pin 4 (reset) go to +VCC; pin 1 goes to ground.
- RA connects VCC to pin 7; RB connects pin 7 to the capacitor node; pins 2 and 6 are joined to the capacitor.
- Pin 5 is bypassed to ground with 0.01 µF.

Output: f = 1.44/[(RA + 2RB)C], duty cycle D = (RA + RB)/(RA + 2RB).

#### Astable multivibrator using BJTs
```text
        +VCC
   +-----+--------+--------+
   |     |        |        |
 [RC1]  [R1]     [R2]    [RC2]
   |     |        |        |
   +--C1-+---B2   B1---C2---+
   |                       |
   Q1, Q2: NPN, emitters to GND
```

Connections: the collector of Q1 is joined through C1 to the base of Q2; the collector of Q2 is joined through C2 to the base of Q1. R1 connects VCC to the base of Q2, and R2 connects VCC to the base of Q1; the emitters are grounded. The output is taken from a collector. f = 1/(0.693 (R1C1 + R2C2)).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In the 555 astable, which pins are joined to the capacitor?
   A. Pins 2 and 6
   B. Pins 4 and 8
   C. Pins 3 and 5
   D. Pins 1 and 7
   Answer: A. Trigger (2) and threshold (6) both monitor the capacitor voltage. (Key point: 555 connections)

2. What is done with pin 5?
   A. Bypassed with 0.01 µF to ground
   B. Connected to VCC
   C. Connected to the output
   D. Left connected to pin 7
   Answer: A. The capacitor prevents noise on the control voltage. (Key point: Pin 5)

3. Where does pin 7 connect?
   A. Junction of RA and RB
   B. Directly to the capacitor
   C. To ground
   D. To the output
   Answer: A. RA connects VCC to pin 7 and RB connects pin 7 to the capacitor. (Key point: Discharge path)

4. In the BJT astable, how is the base of Q2 connected?
   A. To Q1's collector through C1, and to VCC through R1
   B. Directly to ground
   C. To its own collector
   D. To Q2 emitter
   Answer: A. Cross-coupling via C1 and C2 with R1, R2 to VCC. (Key point: BJT AMV)

## Related in Electronics Devices and Circuit

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