Simple Automatic Load Sensing Power Switch

This circuit will automatically switch on several mains-powered "slave" loads when a "master" load is turned on. For example, it will switch on the amplifier and CD player in a stereo system when the receiver is turned on. It works by sensing the current draw of the "master" device through a low value high wattage resistor using a comparator. The output of that comparator then switches on the "slave" relay. The circuit can be built into a power bar, extension cord or power center to provide a convenient set of "smart" outlets that switch on when the master appliance is powered (turn on the computer monitor and the computer, printer and other peripherals come on as well).

Parts

Part	Total Qty.	Description
C1, C3	2	10uF 35V Electrolytic Capacitor
C2	1	1uF 35V Electrolytic Capacitor
R1	1	0.1 Ohm 10W Resistor
R2	1	27K 1/2W Resistor
R3, R4	1	1K 1/4W Resistor
R5	1	470K 1/4W Resistor
R6	1	4.7K 1/2W Resistor
R7	1	10K 1/4W Resistor
D1, D2, D4	3	1N4004 Rectifier Diode
D3	1	1N4744 15V 1 Watt Zener Diode
U1	1	LM358N Dual Op Amp IC
Q1	1	2N3904 NPN Transistor
K1	1	Relay, 12VDC Coil, 120VAC 10A Contacts
S1	1	SPST Switch 120AVC, 10A
MISC	1	Board, Wire, Socket For U1, Case, Mains Plug, Socket

Notes

• This circuit is designed for 120V operation. For 240V operation, resistors R2 and R6 will need to be changed.
• A maximum of 5A can be used as the master unless the wattage of R1 is increased         S1 provides a manual bypass switch.
• THis circuit is not isolated from the mains supply. Because of this, you must exercise extreme caution when working around the circuit if it is plugged in.

Read More

A Simple Hearing Aid

Commercially available hearing aids are quite costly. Here is an inexpensive hearing aid circuit that uses just four transistors and a few passive components.

Hearing Aid Circuit Diagram

Parts:

R1 = 2.2K
R2 = 680K
R3 = 3.3k
R4 = 220K
R5 = 1.5K
R6 = 220R
R7 = 100K
R8 = 680K
C1 = 104pF
C2 = 104pF
C3 = 1uF/10V
C4 = 100uF/10V
C5 = 100uF/10V
Q1 = BC549
Q2 = BC548
Q3 = BC548
Q4 = BC558
J1 = Headphone jack
B1 = 2x1.5V Cells
SW1 = On/Off-Switch

Circuit Operation:

On moving power switch SW1 to ‘on’ position, the condenser microphone detects the sound signal, which is amplified by Q1 and Q2. Now the amplified signal passes through coupling capacitor C3 to the base of Q3.
The signal is further amplified by Q4 to drive a low impedance earphone. Capacitors C4 and C5 are the power supply decoupling capacitors. The circuit can be easily assembled on a small, general-purpose PCB or a Vero board.

It operates off a 3V DC supply. For this, you may use two small 1.5V cells. Keep switch S to ‘off’ state when the circuit is not in use. To increase the sensitivity of the condenser microphone, house it inside a small tube.

Read More

Extend Timer Range For The 555

Anyone who has designed circuits using the 555 timer chip will, at some time have wished that it could be programmed for longer timing periods. Timing periods greater than a few minutes are difficult to achieve because component leakage currents in large timing capacitors become significant. There is however no reason to opt for a purely digital solution just yet. The circuit shown here uses a 555 timer in the design but nevertheless achieves a timing interval of up to an hour! The trick here is to feed the timing capacitor not with a constant voltage but with a pulsed dc voltage. The pulses are derived from the un smoothed low voltage output of the power supply bridge rectifier.

The power supply output is not referenced to earth potential and the pulsing full wave rectified signal is fed to the base of T1 via resistor R1. A 100-Hz square wave signal is produced on the collector of T1 as the transistor switches. The positive half of this waveform charges up the timing capacitor C1 via D2 and P1. Diode D2 prevents the charge on C1 from discharging through T1 when the square wave signal goes low. Push-button S1 is used to start the timing period. This method of charging uses relatively low component values for P1 (2.2 MΩ) and C1 (100 to 200 µF) but achieves timing periods of up to an hour which is much longer than a standard 555 circuit configuration.

Read More

Build 16 Stage Bi Directional LED Sequencer

The bi-directional sequencer uses a 4 bit binary up/down counter (CD4516) and two "1 of 8 line decoders" (74HC138 or 74HCT138) to generate the popular "Night Rider" display. A Schmitt Trigger oscillator provides the clock signal for the counter and the rate can be adjusted with the 500K pot. Two additional Schmitt Trigger inverters are used as a SET/RESET latch to control the counting direction (up or down). Be sure to use the 74HC14 and not the 74HCT14, the 74HCT14 may not work due to the low TTL input trigger level. When the highest count is reached (1111) the low output at pin 7 sets the latch so that the UP/DOWN input to the counter goes low and causes the counter to begin decrementing.

 When the lowest count is reached (0000) the latch is reset (high) so that the counter will begin incrementing on the next rising clock edge. The three lowest counter bits (Q0, Q1, Q2) are connected to both decoders in parallel and the highest bit Q3 is used to select the appropriate decoder. The circuit can be used to drive 12 volt/25 watt lamps with the addition of two transistors per lamp as shown below in the section below titled "Interfacing 5 volt CMOS to 12 volt loads"

Circuit diagram

Read More

Simple Energy Saving Switch Schematic Diagram

Lights do not always need to be on at full power. Often it would be useful to be able to turn off the more powerful lights to achieve softer illumination, but this requires an installation with two separately-switch-able circuits, which is not always available. 

 Energy-Saving Switch Circuit Image

If the effort of chasing out channels and replastering for a complete new circuit is too much, then this circuit might help. Normal operation of the light switch gives gentle illumination (LA1). For more light, simply turn the switch off and then immediately (within 1 s) on again. The circuit returns to the gentle light set-ting when switched off for more than 3 s. There is no need to replace the light switch with a dual version: simply insert this circuit between switch and lamp.

Energy-Saving Switch Circuit Diagram

Parts List:
Resistors:
R1 = 100Ω
R2 = 680Ω
Capacitor:
C1 = 4700µF 25 V
Semiconductors:
D1,D2 = 1N4001
Miscellaneous:
K1,K2,K3 = 2-way PCB terminal
block, lead pitch 7.5 mm
F1 = fuse, 4AT (time lag) with PCB
mount holder
TR1 = mains transformer, 12V @ 1.5
VA, short-circuit proof, PCB mount
B1 = B80C1400, round case (80V
piv, 1.4A)
RE1 = power relay, 12V, 2 x c/o,
PCB mount
RE2 = miniature relay, 12V, 2 x c/o,
PCB moun

How does it work?
Almost immediately after switch-on, fast-acting miniature relay RE2 pulls in, since it is connected directly after the bridge rectifier. Its nor-mallyclosed contact then isolates RE1 from the supply, and thus current flows to LA1 via RE1’s normally-closed con-tact. RE1 does not have time to pull in as it is a power relay and thus relatively slow. Its response is also slowed down by the time constant of R1 and C1. If the current through the light switch is briefly interrupted, RE2 drops out immediately. There is enough energy stored in C1 to activate RE1, which then holds itself pulled in via a second, normally-open, contact. If current starts to flow again through the light switch within 1s, LA2 will light. To switch LA1 back on it is necessary to turn the light switch off for more than 3 s, so that C1 can discharge via R2 and RE1. The printed circuit board can be built into a well insulating plastic enclosure or be incorporated into a light fitting if there is sufficient space.
PCB-Layout

Caution:
the printed circuit board is connected directly to the mains-powered lighting circuit. Every precaution must be taken to prevent touching any component or tracks, which carry dangerous voltages. The circuit must be built into a well insulated ABS plastic enclosure.Link

Read More

protect PC boards from vibration

 These two mounting methods were devised to protect PC boards from vibration when installed in the engine compartment of a car. They could also be used in other applications where vibration is a problem. Method 1 involves rigidly mounting the PC board inside a diecast box and then mounting the box itself to provide vibration isolation. As shown, small grommets are installed in suitably sized holes in the sides of the box.

 

The box is then secured to angle mounting brackets using M4 screws, washers and nylock nuts. Method 2 involves mounting the diecast case onto the chassis of the car and then mounting the PC board as shown, using M3 screws, grommets, hollow spacers and nylock nuts. In this case, the grommets are fitted into suitably sized holes in the PC board itself. Once the nuts are tightened, the PC board should be able to move slightly, relative to the box. If there is not enough space on the board to fit the grommets, then Method 1 is the way to do it.

 

 

 

Author: David Boyes - Copyright: Silicon Chip Electronics

Read More

Simple NiCd Battery Charger

The design of the charger is similar to that of many commercially available chargers. The charger consists of a mains adaptor, two resistors and a light-emitting diode (LED). In practical use, this kind of charger is perfectly all right. Resistor R1 serves two functions: it establishes the correct charging current and it drops sufficient voltage to light the diode. This means that the LED lights only when a charging current flows into the battery. The charging current is about 1/4 of the battery capacity, which allows a slight overcharging, and yet the charging cycle is not too long (4–5 hours).

Circuit diagram:

The value of the resistors may be calculated as follows, for which the nominal e.m.f. and the capacity of the battery must be known. Adjust the output of the mains adaptor to 1.17 times the nominal battery voltage plus 3.3 V, which is the potential across R1. Note that the adaptor must be capable of supplying a current of not less than half the battery capacity. The value of R1 in ohms is equal to 3.3 divided by 1/4 of the battery capacity. The value of the resistors for various battery voltages is given in the Table. The battery capacity is taken as 1 Ah.

The rating of R1 should be 5 W. If the battery to be charged has a different capacity, the theoretical value of R1 in the table must be divided by the battery capacity. Its actual value is the nearest one in the E12 series. For instance, if a 6 V battery with a nominal capacity of 600 mAh is to be charged, the value of R1 must be 20/0.6 = 33R.

Read More