A pulse generator for reversing a d.c. supply at 10 cycles per second for a telephone control circuit uses a relay operated by a transistor either directly or by means of an additional transistor. A timing capacitor is connected to the base of the transistor and the other terminal of the capacitor is switched to ground by the relay after the transistor turns off, and is switched to a charging resistor connected to the power supply after the transistor turns on. When the capacitor is grounded, other contacts of the relay provide a charging path from the power supply through a resistor to the ungrounded side of the capacitor. The first-mentioned charging resistor may be connected to the power supply through R-C filtering. When an additional transistor is used which is of the complementary type relative to the timing transistor, the circuit''s tolerance of power supply ripple can be improved by the addition of feedback through a diode.