Dark Activated Switch

Adjustable Dancing Lights

Here is a simple circuit which can be used for decoration purposes or as an indicator. Flashing or dancing speed of LEDs can be adjusted and various dancing patterns of lights can be formed.

The circuit consists of two astable multivibrators. One multivibrator is formed by transistors T1 and T2 while the other astable multivibrator is formed by T3 and T4. Duty cycle of each multivibrator can be varied by changing RC time constant. This can be done through potentiometers VR1 and VR2 to produce different dancing pattern of LEDs. Total cost of this circuit is of the order of Rs 30 only. Potentiometers can be replaced by light dependent resistors so that dancing of LEDs will depend upon the surrounding light intensity. The colour LEDs may be arranged as shown in the Figure.

A Low Distortion Audio Preamplifier

In an audio amplifier the quality of sound depends upon a number of factors, e.g. quality of active and passive components, circuit configuration, and layout. To an extent, the selection of components depends on the constructor’s budget. The discrete active components like transistors have been increasingly replaced by linear ICs, making the task of designer easier. With the passage of time, the general-purpose op-amps like LM741, which were being used in audio/hi-fi circuits, have become The preamplifier circuit presented here is based on a dual precision op-amp for the construction of a low distortion, high quality audio preamplifier.
A dual op-amp OPA2604 from Burr-Brown is used for all the stages. The FET input stage op-amp was chosen in this context it is worth wile to mention another popular bi-polar architecture op-amp, the NE5534A. It has, no doubt, an exceptionally low noise figure of 4nV/ÖHz but rest of the specifications compared to OPA2604 are virtually absent in this IC. Also This IC is also capable of operating at higher voltage rails of ± 24V (max.). Also its input bias current (100 pA) is many orders lower than its bipolar counterpart’s. This ensures a multifold reduction in noise.

A Highly Efficient DC Lamp Dimmer

The simplest lamp dimmer circuit consists of a rheostat, in series with the lamp, which one may adjust to obtain the required brightness. Such linear regulators are quite inefficient since a lot of power is wasted in them. Moreover, in the rheostat the moving contacts are likely to get damaged in the long run, as its value is frequently adjusted by moving the slider. Such linear control circuits provide an overall efficiency of no more than 50 per cent. This wastage of power can be avoided if one uses pulse width modulation (PWM) which can be made to control an electronic rheostat. The circuit shown here is based on PWM principle. Gate N1 and its associated components constitute an oscillator producing oscillations of approximately 200 Hz with a pulse width of 0.1 ms. This output is fed to transistor T1 for level shifting. At the output of this transistor is a potentiometer VR2, using which a DC component can be added to the pulses emerging from transistor T1. By adjusting this potentiometer/trimmer, one can have a good linear control of the lamp brightness from completely off state to 100 per cent on state. The signal is inverted by gate N2 and fed to MOSFET 12N10. IC CD40106 provides six inverting buffers with Schmitt trigger action. The buffers are capable of transforming slowly changing input signals into sharply defined jitter-free output signals. They are usually used as wave and pulse shapers. IC CD40106 possesses high immunity and low power consumption of standard CMOS ICs along with the ability to drive 10 LS-TTL loads. In this circuit loads up to 24W can be connected between MOSFET drain and 12V supply without using a heatsink. The loads can even be DC motors, miniature heating elements, etc. If one uses a low RDS (on) MOSFET, a higher efficiency can be achieved. By using the components as shown in the circuit, an efficiency of approximately 95 per cent can be achieved. The flexibility of the design makes it possible to change the MOSFET with a similar one, in case of non-availability of 12N10. The circuit by itself does not draw much current when the load is disconnected. Ensure proper ESD protection while handling the MOSFET to prevent damage. Lab note: The circuit was tested using MOSFET IRF640 with RDS (on)=0.18 ohm.

9 Volt 2 Amp Power Supply

There is little to be said about this circuit. All the work is done by the regulator. The 78S09 can deliver up to 2 amps continuous output whilst maintaining a low noise and very well regulated supply.

The circuit will work without the extra components, but for reverse polarity protection a 1N5400 diode is provided at the input, extra smoothing being provided by C1. The output stage includes C2 for extra filtering, if powering a logic circuit than a 100nF capacitor is also desirable to remove any high frequency switching noise.

Unregulated Power Supply

A basic full wave rectified power supply is shown below. The transformer is chosen according to the desired load. For example, if the load requires 12V at 1amp current, then a 12V, 1 amp rated transformer would do. However, when designing power supplies or most electronic circuits, you should always plan for a worst case scenario. With this in mind, for a load current of 1 amp a wise choice would be a transformer with a secondary current rating of 1.5 amp or even 2 amps. Allowing for a load of 50% higher than the needed value is a good rule of thumb. The primary winding is always matched to the value of the local electricity supply.

Shutter Guard

This sensitive vibration sensor is exclusively made for shops to protect against burglary. It will detect any mechanical or acoustic vibration in its vicinity when somebody tries to break the shutter and immediately switch on a lamp and sound a warning alarm. A 15-minute time delay after switch-on allows sufficient time for the shop owner to close the shutter.

The front end of the circuit has a timer built around the popular binary counter IC CD4060 (IC1) to provide 15-minute time delay for the remaining circuitry to turn on. Resistors R3 and R4 and capacitor C2 will make Q9 output high after 15 minutes. Diode D1 inhibits the clock input (pin 11) to keep the output high till the power is switched off. Blinking LED1 indicates the oscillation of IC1. The high output from IC1 is used to enable reset pin 4 of IC2 so that it can function freely. Transistor T1 amplifies the piezo-sensor signal and triggers monostable IC2. The base of transistor T1 is biased using a standard piezo element that acts as a small capacitor and flexes freely in response to mechanical vibrations so that the output of IC2 is high till the prefixed time period.
In the standby mode, the alarm circuit built around IC3 remains dormant as it does not get current. Timing components R8 and C6 make the output of IC2 high for a period of three minutes.
When any mechanical vibration (caused by even a slight movement) disturbs the piezo element, trigger pin 2 of IC2 momentarily changes its state and the output of IC2 goes high. This triggers triac 1 and the alarm circuit activates. Triac BT136 completes the lamp circuit by activating its gate through resistor R9. IC UM3561 (IC4) generates a tone simulating the police siren with R11 as its oscillationcontrolling resistor. Zener diode ZD1 provides stable 3.1V DC for the tonegenerating IC.
Assemble the circuit on a generalpurpose PCB and enclose in a suitable, shockproof case. Connect the piezo element to the circuit by using a single-core shielded wire. Glue a circular rubber washer on the fine side of the piezo element and fix it on the shutter frame with the washer facing the frame so that the piezo element is flexible to sense the vibrations. Fix the lamp and the speaker on the outer side and the remaining parts inside the case. Since triac is used in the circuit, most points in the PCB will be at mains lethal potential. So it is advised not to touch any part of the circuit while testing.