central-heating monitor - with LED indication

with LED indication One of the largest annual expenses for most home owners is the fuel for the central heating. Apart from keep- ing the house temperature as low as comfort will allow there is not very much we can do about this bill. We just set the temperature and let the room thermostat and central heating boiler get on with it. The circuit shown here is designed to go one step further by allowing us to see what the boiler and thermostat are doing at all times. The circuit indicates various con ditions by means of three LEDs. They show when the thermostat re- quests more heat (D1) and whether the boiler is then on (D2) or off (D3). We will see how this is achieved by referring to the diagram of figure 1, and beginning with the request for more heat. The relevant LED, D1, will not light if the input of N3 is "1". This is the case if there is 24 V a.c. across the open contacts of the room ther- mostat (RT). This voltage is rectified by R15, D7, R7 and C4 before being passed to N3...