Patent GB-656275-A (GB)
Title: Improvements in or relating to electronic timing devices
Inventor: HERCOCK ROBERT JAMES
IPC: H03K17/288
Filed: 1948-07-27
Abstract: 656,275. Valve relay circuits. ILFORD, Ltd. July 27, 1948, No. 20000. [Class 40 (vi)] In an electronic timing device particularly for photographic exposure intervals, comprising a condenser, resistor, and gas-filled relay, the condenser is charged through the resistor, or after charging is discharged through the resistor and the gas-filled relay fires when the charge on the condenser has reached or fallen to a predetermined value to determine a time interval during which a lamp glows, means being provided for varying the charge on the condenser so that variations in the intensity of light from the lamp due to variations in the supply voltage are compensated for by corresponding variations of the time during which the lamp glows. As shown in Fig. 5, on closing S the lamp L lights and a potentiometer R6, R7, which may include a stabilizing neon V2, is connected across the D.C. supply, the condenser C1 charging through resistors R3, R5, until the cold-cathode gasfilled relay V1 fires. This energizes the relay A/2 which closes the contacts a/1 to discharge the condenser and opens contacts a/2 to disconnect the lamp L, any increase in voltage across R7 due to an increase in mains voltage causing condenser C1 to reach its critical firing potential in a shorter time interval. In a modification of Fig. 5, Fig. 6 (not shown), to increase the life of the valve V1 the relay is provided with extra contacts which connect a resistor in place of V1 so that the valve only conducts to energize the relay. In a further arrangement, Fig. 9, when the timer is connected to the mains the valve V1 fires and energizes relay A/2, opening contact a/2 to disconnect the lamp and shorting resistors R3, R4, through contact a/1, so that condenser C1 is initially charged to the anode-cathode potential of V1. To initiate an exposure the switch S is depressed and released after a short delay and contacts a/2 close the lamp circuit, contact a/1 returning to the position shown. The condenser C1 then discharges through resistors R3 and R4 in parallel with R7 and the grid-cathode space, until the grid of V1 reaches the triggering potential and the relay operates to terminate the exposure, mains voltage changes during the timing inter. val altering the effective value of the firing potential. In a modification giving improved compensation, Fig. 12 (not shown), a valve VI is used having a grid-cathode characteristic exhibiting a negative slope, the condenser C1 being initially charged across the grid-cathode discharge of the valve operating at that point. In Fig. 15 (not shown), the condenser C1 is initially charged in a bridge circuit having a non-linear resistor in one arm, the condenser being connected to one diagonal and the supply voltage to the other as in Fig. 14 (a) (not shown), and is then discharged until the gas-discharge tube strikes. Alternatively, as in Fig. 14 (b) (not shown), and Fig. 17 (not shown), the con. denser may be charged in a similar bridge circuit using the gas-discharge tube in place of the non-linear resistor. In these circuits the compensation is mainly effective for the mains voltage value at the initiation of the time interval. In Fig. 19 application of the supply voltage causes the valve V1 to fire and energize relay A/3, closing contact a/2 and opening contacts a/1 and a/3, the condenser then being connected in a bridge circuit as in Fig. 14 (b) (not shown), and having an initial' charge dependent on the mains voltage. On momentarily opening the push-button ST the valve de-ionizes, the relay contacts return to the position shown and the condenser continues to charge to a potential dependent on the mains voltage until the valve strikes and terminates the exposure. This circuit may be improved, Fig. 20 (not shown), by making a/2 a changeover switch so that C1 may be disconnected from the junction of R2 and R3. As an alternative to switching a lamp on and off the timing device may control an on-off shutter in association with a lamp. Specifications 508,449, 613,565, 614,472 and 628,271 are referred to.