Relay Operated Standby Switch

In this Instructable we will go over how to construct a Relay operated Standby Switch. Though its intended purpose is to be used in a guitar amp, one can adapt the circuit to any scenario where a time-delayed voltage source is desired. The purpose of applying this design to a guitar amp is to keep high DC voltage off of the panel and out of the hands of the user, making the system much safer to operate. It also affords the designer the ability to use a more commonplace 120VAC rated switch rather than having to acquire its high voltage counterpart. Using the relay allows for only 5-10 VDC to be at the switch instead of >400VDC. A Circuit Explanation is detailed below.

Circuit Explanation

In this Circuit V2 (high voltage) is controlled by V1 (low voltage) using 1 NPN Transistor (Q1) and a 5-10VDC Relay. The emitter of (Q1) is connected to the positive lead (+) of the Relay and the negative lead of the Relay to ground. One lead of the Standby Switch is connected to the positive voltage rail (5-10v) along with the collector of the NPN transistor. The other lead is connected to a 2.2k resistor (R1) which then connects to the base of the NPN transistor. The positive (+) terminal of an electrolytic capacitor (C1) is also connected to the base of this transistor then the negative (-) terminal to ground. The value of (C1) will be used to establish how much time it takes to close the normally-open Relay. When the Standby switch is closed the low voltage will start to travel to the collector of (Q1) and to the base of (Q1) via (R1) and (C1). Once (C1) is completely charged, voltage in the base-emitter junction of (Q1) will start to flow thereby allowing positive voltage to close the Relay. V2 will now travel across the Relay. Decreasing the value of (C1) will cause the Relay to close sooner. Increasing the value of (C1) is cause the Relay to close later. With a 2.2k Resistor and a 470uf Capacitor, it takes about 1sec for (C1) to charge and close the Relay.

(1) Relay

(1) NPN Transistor

(1) Resistor (22k in this case.)

(1) Capacitor (470uf in this case)