Time Delay Touch Switch

Fitting switches which turn off automatically after a preset time in these locations can reduce the electricity consumed quite significantly. A single 100W bulb left burning all night (say eight hours) would consume 800W/hrs. The same bulb fitted with a switch which turned off automatically after say five minutes would consume only 83W/hrs, even in the unlikely event that it were to be activated ten times during this period.

Triac Control
There are two major problems to solve when designing a touch switch to replace a conventional mechanical light switch and these concern the power supply and noise immunity.

The main switching element used to control the light is a triac, a semiconductor a.c. switch. This is normally in its off or non-conducting state but can be switched into conduction by means of a small trigger current fed to its gate terminal. Once triggered it will remain conducting until the current through it drops below a certain value (known as the holding current). This occurs normally on a.c. supplies when the voltage drops to zero at the end of every mains half-cycle, and the triac must be retriggered again if the light is to remain on.

As with any switch, the voltage across a triac when it is in its off state will be the supply voltage (in this case 230V a.c.). When the triac is on, the voltage across it will be around 1V a.c., depending on the current. The triac therefore dissipates very little power so no heatsink is required.

MAINS Failure Alarm

MAINS failure alarms are often employed in situations where the removal of the mains supply from a piece of equipment can have adverse or even disastrous consequences.

Very often, the “failure” may simply be the result of the switch on the outlet socket being operated inadvertently and this may have no immediately noticeable effect on the equipment. The results of the mistake may only become evident a few hours or even days later, when it is too late.

The circuit described here overcomes all of these problems at a stroke and also does away with the need for mains plugs, or indeed any connections to the mains at all. It does so by monitoring the electric field which exists around a cable connected to the a.c. supply (whether it is carrying a current or not).

By placing it on or near to the cable of the appliance to be monitored, it will also sound the alarm if the fuse in the plug blows, the outlet is switched off or the plug disconnected. It will only fail to detect the situation where the equipment itself has been switched off via its own built-in switch. However, since many appliances such as freezers do not have on/off switches, this is not really a problem.

The circuit is extremely simple and inexpensive to build. Its simplicity and lack of any specialized components should make it attractive to many constructors who will probably already have most of the components to hand. The absence of any mains connections should make it an ideal project for a beginner.

As with many simple circuits, however, the advantages and possible uses take longer to describe than the operation of the circuit diagram which is shown in Fig.1.

Voltage Booster – High Voltage from a 5V Supply

Digital circuits operating from 5V regulated supplies are common but occasionally a higher voltage is required, perhaps for a bio-medical circuit, or for liquid level measurement or for monitoring high resistance contacts. For such circuits a means of generating a high voltage from the 5V supply can be a solution. Diode/capacitor multipliers can offer advantages over switched-mode circuits, since they do not use inductors, are easier to design and troubleshoot and often generate less radiated interference.

The principle of the voltage multiplier is fairly well known. A capacitor is used with a square wave drive signal to “pump” current through a pair of diodes, roughly doubling the supply voltage. A series of such stages can be cascaded to raise the voltage in multiples of the supply, but it is possible to improve efficiency and reduce the number of stages by using two driving signals with opposite phases. However, each diode incurs a drop of about 0.6V so with two diodes per stage, and with an initial supply of just 5V this becomes significant, leading to poor efficiency and an impractical number of stages.

These problems are overcome in the Voltage Booster circuit design of Fig.1 by increasing the voltage before multiplication with IC1, an SI7660 “negative rail generator” (not the ICL7660 – ARW). The additional negative supply is generated very efficiently since switching is performed not by diodes but by CMOS switches in the IC, which cause almost no voltage loss at low currents.

PIC Based Air Quality Monitor

Idealy if you have a combustion heater in your home, there should be some means of monitoring the air quality. This is where the Air Quality Monitor comes in, it measures both carbon dioxide and carbon monoxide levels, and displays the results on LED bargraphs. If the concentration of either of these gases rises above a preset level, a loud alarm will sound, which means that you should turn off the heater and open the room up to fresh air.

Each bargraph comprises eight LEDs that light invidually to show eight distinct levels. In between values are displaed by lighting two adjacent LEDs. This gives a total of 15 levels that can be displayed. The four lower LEDs are green, followed by two orange and then two red LEDs. An automatic dimming circuit ensures that the LED displays are not too bright at night.

In addition, the alarm sounds if any of the top three LEDs light in either display. There are three alarm levels:

1. Main alarm sounds if the top LED lights. This consists of a 64ms-long 4kHz tone that repeats every 0.5s. 
2. Less urgent alarm sounds if the second top LED is lit (top LED off). This alarm gives a 32ms-long 4kHz ‘chip’ every four seconds (4s). 
3.   Warning alarm sounds if the third top LED is alight. This alarm mode gives a brief 16ms 4kHz ‘chirp’ every 16 seconds (16s). 

An internal fan at one end draws air through the box so that the internally mounted CO and CO2 sensors are presented with a continuous sample of the air that’s being monitored. Power for the unit comes from a 12V DC 500mA plugpack.

12V 10A High current Power Supply with battery backup

The full circuit diagram of the 12V 10A Power Supply is shown in Fig.1.Power of about 18V to 25V is applied to screw terminals pins 1 (+V) and 2 (–V).Although the power supply was originally designed to run packet radio transceivers, the unit is not just confined to this type of radio. In fact, any radio can be used with this power supply. It can also be used as a main source of power; ie the battery, or used as a standby source of power in the event of a power failure. There would be a float charge for the battery when mains voltage is applied, and the battery can be relied on to supply current to equipment when the mains supply fails.

The input current to the circuit is limited by a 5A fuse (FS1) for protection purposes. Relay RLA is a small ‘homemade’ reed type that is set to close the contacts when the current drawn through this relay coil is in the order of about 1.5A. When this occurs, RLA’s contacts close, current is drawn through resistor R1, LED1, R2 and onto R3. The voltage across resistor R3 is sufficient to turn on FET TR2, which supplies power to a 12V cooling fan.