TAP thieves on the head

once the ignition is switched off and the ignition key removed, the second, concealed switch must also be in the off position, otherwise the anti-theft circuit is pointless. However it is all too easy to forget to operate the concealed switch when leaving one`s car in a hurry. The circuit shown in the accompanying diagram represents an attempt to get round both these problems. To start the engine the ignition switch, S1, is first closed. This however fails to energise the ignition coil, since the contacts of the relay, re/a, which is inserted in the ignition lead, remain open. If however the touch contacts are bridged with the finger, a small base current will flow through T1, turning on this transistor ~d the Darlington pair, T2 and T3. As a result, the relay, re, pulls in, and once the contact re/b is closed, the relay will remain in that state. The engine can now be started normally. When the ignition switch is opened, the relay will automatically drop out, thus `re-arming` the anti-theft facility.