Linking gates with capacitors
Fig. 45.30 shows a single transistor NOT gate or inverter. The input chain to it contains a capacitor. When switches S, and S2 are open, the transistor base is linked to the power supply positive line through the resistor R, the transistor is on and its output voltage is low. If switch S, is closed, charge will flow onto the plates of the capacitor. The energy from the power supply is being stored on these two plates. The
input voltage to the transistor will increase as the charge stored on the capacitor and the voltage across it increase.
The capacitor will charge up fully, and the voltage across it will increase. When this voltage is sufficiently high, the transistor will switch on again as current flows into the base. The time T taken for this will depend on the value of the resistor R, and the capacitor C. For the 220 jlF capacitor and a 100 k!l resistor T= R x C = 220 X 10-6 F x 100 X 103!l = 0.22 s
This is not exactly the time it takes for the transistor to switch off again, but it does give an indication of the time scale of the switching operation. The system can be ‘reset’ by closing S2 so that the capacitor discharges again. With two cross-connected switches, each transistor can switch on another transistor as it is itself switched off. Fig. 45.31 shows such an arrangement, an astable multivibrator.
The multivibrator does have two output states, but they are not stable. The times
spent in the two states depend on the time constants of the two resistor-capacitor
chains (R3 x C\) and (R4 x C2). The smaller these time constants, the quicker the
states will change, and the higher the frequency of the pulses produced.
Fig. 45.32 shows a practical version of the astable circuit. The load resistors have been replaced by filament bulbs. With the component values shown, the time constant
of the RjC pairs is (100 k!l x 22 uf’), or 2.2 s. These various transistor circuits, logic gates, and bistable and astable multivibrators
can be used together. Fig. 45.33 shows one example in ‘block’ form-the boxes represent the different electronic systems. The two resistors shown represent R3 and R4 from Fig. 45.31. They can be varied to alter the timing of the pulses from the slow astable. The whole system turns on and off three coloured LEDs in sequence.
These glow red, amber and green, and this circuit gives a traffic-light sequence for the three colours.
The astable multivibrator operates the amber light. This keeps flashing on and off, as the middle row of Fig. 45.34 shows. The red light changes at half this frequency. This is operated by the bistable, being switched on and off by falling pulses at the
output of the astable unit. Finally, the green light is on when neither red NOR amber is on. So it can be operated by linking these two outputs, via a NOR gate, to the green LED.
This circuit will only work well if the logic gates can provide enough current to operate the indicators. With light-emitting diodes, this is possible. With higher powered lamps, electromagnetic relays (Fig. 45.18) could be used.