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.

Universal Remote Control Receiver

The circuit diagram shown in Fig.1 uses a PNA4602M IR detector to receive an IR signal from  the remote control. It’s a neat device, which contains an IR receiver, amplifier and demodulator together in a single 3-pin package, and it’s normally used in circuits where you want to decode an IR remote control signal. 

The way the circuit (Fig.1) works is as follows. When there is no IR coded signal present, the output pin of IC1 remains high. This high signal is fed to the trigger input of the 555 timer (IC2), which being configured as a monostable timer, prevents the timer operating. 

Whenever you briefly press any key on the old remote control its IR signal is received by IC1 and output pin 1 produces a train of fast moving high and low pulses, which mimic the IR signal code sent by the remote control. We are not interested in this code, but as soon as the signal switches low it triggers the monostable timer IC2 and its output pin 3 goes high for a short period of time, set by resistor R2 and capacitor C2.

LDR Based Water Pump Controller

Here is a simple solution for automatic pumping of water to the overhead tank. Unlike other water-level indicators, it does not use probes to detect the water level and hence there is no probe corrosion problem. It has no direct contact with water, so the chance of accidental leakage of electricity to the water tank is also eliminated.

Two important advantages of the circuit are that the water level never goes below a particular level and no modification in the water tank is required. Fig. 1 shows the circuit of the water pump controller. The circuit uses an LDR-white LEDs assembly to sense the water level. It forms a triggering switch to energise the relay for controlling the pump. The LDR-LEDs assembly (shown in Fig. 2) is fixed on the inner side of the cap of the water tank without making contact with water. The light reflected from the water tank is used to control the resistance of LDR1.