A two-terminal switch circuit (1) for inclusion in series with a load (20) across a voltage supply (63) for periodically energizing the load (20) from the supply (63) comprises a controllable switch (4) connected between the terminals (2,3), a resistance (8) and a capacitor (9) connected in series between the terminals (2,3), and a voltage threshold responsive arrangement (26) the output signal/input signal characteristic of which exhibits hysteresis. The voltage threshold responsive arrangement (26) has a signal input (27) connected to the common point (12) of the resistance (8) and the capacitor (9), and a signal output (28) connected to a control input (13) of the controllable switch (4). A power supply input (29) of the voltage threshold responsive arrangement (26) is also connected to the common point (12) of the resistance (8) and the capacitor (9) so that the voltage threshold responsive arrangement (26) is powered from across the capacitor (9). In operation the threshold responsive arrangement (26) closes the switch (4) when the capacitor (9) has charged to above a first threshold voltage and opens the switch (4) when the capacitor (9) has thereafter discharged to below a second threshold voltage which is lower than the first threshold voltage. The cycle then repeats. The part of the switch circuit (1) which excludes the capacitor (9) may be constructed as a unitary circuit component having three terminals. The load (20) may comprise direction indicator lamps (21, 22, 23, 24) of a road vehicle.