Protective components

Diodes connected in series with base or emitter of the transistors is required to prevent base-emitter junction being driven into the reverse breakdown when supply voltage is in excess of the Veb breakdown voltage, normally around 5-10 volts for the general purpose silicon transistors. In the monostable configuration, only one of the transistors needs protection.

Figure 1: Basic  BJT monostable multivibrator.

Figure 2: Basic  BJT bistable multivibrator.

Non-electronic astables

Astable oscillators are typically thought of as  electronic circuits, but need not be.  Bimetallic strips are  used  which  switch  an  electric  current  on  as  they  cool and  off  as  they  heat-flashing Christmas and car  indicator lights may use this mechanism.

Monostable multivibrator circuit

When triggered by an input pulse, the monostable multivibrator will switch to the unstable position of it for a period of time, and then return to its stable state. The time period monostable multivibrator remains in the unstable state can be given by t = ln(2)R₂C₁. If repeated application of input pulse maintains the circuit in unstable state, it is known as retriggerable monostable. If further trigger pulses do not affect period, circuit is a non-retriggerable multivibrator.

Bistable multivibrator circuit

Suggested values:

• R₁, R₂ = 10 kΩ
• R₃, R₄ = 10 kΩ

The latch circuit is similar to the astable multivibrator, except that there is no charge and discharge time, because of the absence of capacitors. Therefore, when the circuit is switched on, if Q₁ is on, its collector is at 0 V. As a result, Q₂ gets switched off. This results in over half + V volts being applied to R4 causing current into base of Q₁, hence keeping it on. Therefore, the circuit remains stable in a  single  state  continuously.  Likewise,  Q₂  remains  on  continuously,  if  it occurs to get switched on first.

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