Aussiemike in Kenya – who suggested that light from a mobile phone’s display screen could be used to trigger a switch. When a brief (free) phone call illuminates the display – the light from the display operates a 4017 based Toggle Switch. And any small push switch – connected in parallel with the LDR – will provide optional manual control.
The switch can be operated remotely – from any phone. The call should be kept below about 5 seconds. With the phone’s backlight set to turn off 10 seconds after the call ends – the screen will go dark after a maximum of about 15 seconds. This is more than long enough to toggle the switch. And – if you know the state of the relay before the call – you’ll know its changed state – after the call.
But lets say you don’t know – or you’ve forgotten – the current state of the relay. Then what? The solution is to begin by resetting the circuit. If you extend the length of the call to at least 30 seconds – the 4017 will reset. The state of the relay before the extended call – doesn’t matter. After the extended call – it will be de-energized. And the next (short) call will cause it to energize.
This switch will have many uses. And not all of them will involve distance. So I’ve included a number of signals – to indicate the changing state of the relay. When the relay energizes – the red LED lights – the green LED flashes twice – and the Buzzer gives two short beeps. When the relay de-energizes – the red LED turns off – the green LED flashes four times – and the Buzzer gives four short beeps.
The LEDs and Buzzer are not necessary to the operation of the circuit. You can leave them out if you wish – together with their associated components. That is – you can leave out all of the components – coloured purple in the schematic diagram.
Copyright Ron J