GB869233A - A transistor free-running multivibrator - Google Patents

Abstract

869,233. Road traffic and marine signalling- apparatus. NATIONAL RESEARCH COUNCIL. Dec. 10, 1957 [May 6, 1957; Aug. 28, 1957], No. 38385/57. Class 118. [Also in Groups XL (b) and XL (c)] A free-running multivibrator for operating the flashing lamp of a marine or aircraft signalling device comprises two transistors, each having its collector connected to the opposite base through a circuit comprising a resistor and a capacitor in series. In Fig. 5, the collector and base electrodes are cross-coupled through capacitors C1 and C2 so that the transistors conduct alternately, capacitor C1 being larger than C2 so that transistor TR2 is conductive for a longer period than TR1. Resistors R2 and R5 are connected in series with the capacitors to limit the current flowing into the base electrode of the transistors and the normal base resistors R3 and R4 are connected to the collector electrode instead of to the collector supply terminal to stabilize the oscillation of the circuit with changes in temperature. The potential at the collector or transistor TR2 is amplified by transistors TR3 and TR4 and applied to a number of transistors TR5 to TRn in parallel acting as switches to control the current in a lamp 12 so that a flashing signal is produced. A small battery 26 is provided to ensure that the output transistors are fully cut-off by the drive from transistor TR4, the battery being charged when transistor TR4 is conducting and discharged through resistor R13 when it is not conducting. The capacitors or the resistors of the multivibrator circuit may be variable to vary the frequency of operation and the arrangement can be such that the lamp is on or off for the longer period. A lightsensitive cell 30 is connected between the basis of the transistor to switch the circuit to the off condition of the light during daylight hours. This cell could alternatively be connected between the lower terminals of capacitors C1 and C2 or two cells could be provided, each connected across one of the resistors R2 and R5 (Figs. 7 and 8, not shown). Alternatively, the cell could be replaced by a heat-sensitive device such as a thermocouple. The circuit may alternatively be rendered inoperative during daylight hours by a photo-sensitive resistor connected in series across the supply with a relay which interrupts the supply to the circuit (Fig. 6, not shown). Alternatively, when a relay is used in place of the transistor switching circuit, the photo-sensitive resistance could be connected in series or parallel with the transistor operating the relay (Fig. 2, not shown). The circuit could alternatively control gas vapour liquid or solid supply to a lamp or could operate a stepping or continuous motor.