| 摘要 |
<p>A signalling device, such as a fire detecting device, has a head (2) which is removable from a base (1). The head has circuit means (4,5,6) for sensing and signalling a change in given condition. The base is connected to supply lines and has fault signalling means (3) for generating a fault signal due to head removal or some other device fault. As the same signal is generated in each case, the invention provides an economic, simple and reliable solution to the problem of signalling fault conditions. Preferably, the fault signalling means includes a first circuit (3) in the base (1) which responds to an output from a second circuit (7) in the head (2) so that, if the head is not fitted to the base, the first circuit (3) responds to the absence of said output from the second circuit (7) in order to apply the fault signal to the lines.</p> |
