GB1367641A - Proximity alarm control device - Google Patents

Abstract

1367641 Detecting voltage: proximity detectors CRANE PRODUCTS MFG CO Inc 1 July 1971 [19 April 1971] 30834/71 Heading G1N and G1U For a proximity alarm which gives warning when an object enters an electrostatic field, an alarm control device comprises an electrically conductive casing containing a signal control circuit which is encapsulated and plugged into the casing, a TEST circuit having a first condition for applying an induced signal to the signal control circuit and a second condition for testing the operation of the control device, the signal control circuit amplifying the induced signal and having an oscillator for providing a signal in response to the amplified signal to operate an alarm circuit and a power supply voltage regulator. As described, the sensor 11 may be an antenna mounted on the boom of a crane or on a fork-lift or ladder truck. When the antenna is moved into the proximity of a power line, the induced voltage is reduced by means of an adjustable capacitive divider 14 and a potentiometer 23 and passed to the gate of a field-effect transistor 25, feeding a two-stage amplifier comprising transistors 26, 27. Capacitors 62, 64, 67 reduce the sensitivity to radio frequency signals. The amplified signal is rectified by a half-wave voltage doubler 28, 29, 31 and used to reduce the current in transistor 32 so as to allow a uni-junction transistor 37 and capacitor 36 to oscillate at a frequency which is a function of the signal strength and having a maximum value of five pulses per second. After stretching by a transistor 38 and capacitor 40 and amplification by transistors 43 and 45, the pulses are used to operate a relay 46, arranged to provide audible clicking and to flash a lamp 81. Testing. A TEST button 12 operates contacts 13a... 13c which check the continuity of the antenna feeder 11, check the power supply by means of lamp 80 and check the oscillator 37 by short circuiting the base of transistor 32. Construction. The circuit shown in Fig.5 may be encapsulated in resin and mounted together with the switching arrangement shown in Fig. 6, in an aluminium casing Figs. 1-4, (not shown), the circuits shown in Figs. 5 and 6 being coupled by a multiple plug and socket 24a, 24b.