Circuit diagram:
Parts:
R1______________1K 1/4W Resistor R2____________220K 1/4W Resistor (Optional, see text) R3______________2K2 1/4W Resistor (Optional, see text) C1___________1000µF 25V Electrolytic Capacitor (See Notes) C2____________100nF 400V Polyester Capacitor (Optional, see text) D1__________1N4002 100V 1A Diode D2_____________5mm. Red LED P1_____________SPST Pushbutton BZ1___________Piezo sounder (incorporating 3KHz oscillator) (Optional, see text)
Comments:
This very simple and self-powered device was conceived to allow a person to monitor if someone has rung his home door-bell when he was out.
As most door-bells use 12Vac supply, the circuit must be simply connected to the two door-bell-coil leads.
When the door-bell is activated, an ac voltage of about 10 - 16V is developed at its leads, so it is rectified by D1 and charges C1.
Though this device was primarily intended to be used when a person leaves the house for a few hours or a week-end, a number of tests has proven that a minimum of 15 days "memory" can be guaranteed, even using cheap capacitor types.
To know if the door-bell has rung, you must simply push P1: if the event occurred, LED D2 will illuminate and will fade slowly to the off-state in some seconds. This operation will reset the circuit also.
The LED can be substituted or supported by a small Piezo sounder (incorporating 3KHz oscillator)
A small number of door-bells powered by 230 or 115Vac can be found. In this case, R2, R3 and C2 must be added to the input of the basic circuit, allowing about 15Vac to develop across R3, the voltage-drop provided by C2 reactance.
The mains supply operation facility, allows further development of the circuit purposes.
In fact, almost any mains supplied apparatus can be monitored, e.g. household appliances, computers, radio and television sets, Hi-Fi systems, lamps etc., provided the circuit can be inserted after the main on-off switch.
Notes:
- The added high-voltage circuit formed by R2, R3 and C2 was designed for 230Vac operation. If your ac mains is rated at about 115V, you must change C2 value to 220nF 250V. No other changes are required.
- In most cases, a 470µF 25V value for C1 would suffice.
- Warning! If the circuit is connected to 230Vac mains, some parts in the circuit board can be subjected to lethal potential! Avoid touching the circuit when plugged and enclose it in a plastic box.
