<p>The system is particularly suitable for controlling a solenoid valve SV1 having two stable operating positions in accordance with programmed 'on' and 'off' periods set at a 24 hour timer 10. The valve typically in one position connects a gas supply to a regulator ensuring the latter then supplies gas at a higher pressure; and in the other position connects the regulator to atmosphere through a vent stack so that the regulator then supplies gas at a lower pressure. The system is powered by 3 volt batteries B1, B2 and has extremely lower current consumption giving a battery life of 5-6 years. The solenoid 18 is energised briefly by voltage pulses fed from a capacitor C5 via a reversing switch 16. Automatic battery voltage checking is incorporated.</p>
