describes how to build a "soft touch" switch. By "soft touch" we mean
that you have to push once to set device ON and push again to set device
OFF. This kind of switch works by latching a relay to ON state with push
of a button and with another push latch is released. It is working like
flip-flop states. In that way you can control power to a device using
one push button.
is build around a 555 timer configured in a way that let it latch on one
state and an action is required to change state. The circuit is
powered from +5V and there are connectors to connect controlling device.
Inspiration from this project and circuit is found here:
http://todbot.com/blog . We added a relay on output, an indicator
led as well as connectors for power, external push button and relay
Download Schematic in Eagle
format of PDF format
Schematic for this circuit can be seen above. The
circuit toggles a relay when button S1 is pressed. Operation of this
circuit is simple. Pins 6 and 2 of 555 timer are at half power voltage.
When output pin 3 is high then capacitor C1 is charged and when it's low
capacitor is discharged. When button is pressed capacitor voltage
appears on pin 6 and 2 and output pin 3 changes state as well as
capacitor voltage changes. So when output is high capacitor has +5V.
When we press the button pin 3 goes low and capacitor goes 0V, when we
press button again output goes high again, so we have toggle
functionality. When output is high transistor T1 is conducting current
and relay is latched, when low relay is released. Diode D1 is used to
protect transistor from back voltage generated when relay goes off.
A disadvantage of this circuit
is that when we connect power to the circuit relay is engaged. A
solution to this can be found on the second reference website below.
Download PCB in
Eagle format of
Circuit is in OFF state
Push button is pressed and circuit goes to ON
state. Relay switch on a high power LED
Download Schematic and
PCB Eagle files