## Saturday, February 26, 2011

### Light and Dark Dependent Circuir

This is the circuit of a simple buzzer that is activated by darkness, the buzzer is off when there's light and on when it is dark. A general-purpose operational amplifier, the 741, is used as a comparator that determines whether it is dark enough to turn on a self-oscillating piezoelectric buzzer.

Its inverting input is connected to a photoresistor, a component whose resistance decreases as more light shines on it.  Its non-inverting input, on the other hand, is connected to an almost fixed voltage, i.e., a proportion of the supply voltage as set by timmer resistor R2.

Figure 1.  Darkness-Activated Buzzer Circuit Diagram.

If there is ample light shining on the photo-resistor, the buzzer is quiet. As less light shines on the photo-resistor, its resistance increases and causes the voltage across R1 to decrease.  At a certain level of lighting, the voltage across R1, which is also the voltage at the inverting input of the 741, becomes smaller than the voltage at the non-inverting input. At this point, the 741 is triggered to output a 'high' level, turning on Q1.  Q1 then activates the self-oscillating piezoelectric buzzer.

### Frequency Meter Circuit

This is the circuit of frequency meter. It measures the frequency of the input signal in terms of current passing through an ordinary ammeter. The main component of this circuit is the 555, a versatile, general-purpose timer IC. In this circuit, the 555 is configured as a monostable multivibrator that outputs a single pulse at pin 3 every time the input signal at pin 2 goes 'low'.

The width of the output pulse generated by the 555 is defined by R3 and C3.  The larger the output pulse width, the longer is the time that pin 3 is high. Since pin 3 doesn't sink current when it is high, the current passing through the ammeter is higher when pin 3 is high.

This means that the larger the values of R3 and C3 are, the larger is the output pulse width, and the higher is the current passing through the ammeter for a given input frequency.For any given value of R3 and C3, on the other hand, the current passing through the ammeter increases as the input frequency increases.

This is because pin 2 is 'retriggered' more often as the input frequency increases, decreasing the total duration wherein the output is 'low'.  Thus, the ammeter dial in this circuit would be a good indicator of the relative frequency of the input signal at pin 2. Good choices for the values of R3 and C3 to match the intended application as well as careful calibration through R5 are required to make this simple circuit work properly.

### Simple AM Transmitter Circuit

AM transmitter circuit that can transmit your audios to your backyard.This circuit is designed with limited the power output to match the FCC regulations and still produces enough amplitude modulation of voice in the medium wave band to satisfy your personal needs. You will love this.

The circuit has two parts , an audio amplifier and a radio frequency oscillator. The oscillator is built around Q1 (BC109) and related components. The tank circuit with inductance L1 and capacitance VC1 is tunable in the range of 500kHz to 1600KHz.

These components can be easily obtained from your old medium wave radio. Q1 is provided with regenerative feedback by connecting the base and collector of Q1 to opposite ends of the tank circuit. C2 ,the 1nF capacitance , couples signals from the base to the top of L1, and C4 the 100pF capacitance ensures that the oscillation is transfered from collector, to the emitter, and through the internal base emitter resistance of the transistor Q2 (BC 109) , back to the base again.

The resistor R7 has a vital part in this circuit. It ensures that the oscillation will not be shunted to ground trough the very low value internal emitter resistance, re of Q1(BC 109), and also increases the input impedance such that the modulation signal will not be shunted to ground.

Q2 is wired as a common emitter RF amplifier, C5 decouples the emitter resistance and unleashes full gain of this stage. The microphone can be electret condenser microphone and the amount of AM modulation can be adjusted by the 4.7 K variable resistanceR5.

### Circuit of Light Dimmer

The dimmer presented here may be built into a wall-mounted box containing the light switch. It is intended for use with 220 V incandescent lamps only. When it is fitted, and the light is switched on, the lamp does not come on fully for about 400 ms.

When the light is switched off, it stays on unchanged for about 20 s, and then goes out gradually. This has the advantage that it is not immediately dark when the light is switched off.

When light switch S1 is turned on, capacitor C2 is charged via R1, C1 and bridge rectifier D1–D4. Zener diode D5 limits the potential across C2 to about 15 V. After a short while, diode D6 lights, whereupon a potential difference ensues across light sensitive resistor R3, which is sufficient to trigger triac Tr1.

The light then comes on. When the light switch is turned off, C2 is discharged via P1, R2 and D6. When the potential across C2 drops, the brightness of the LED diminishes, so that the p.d. across R3 also drops. The increasing resistance of R3 effects phase angle control of the triac so that the light is dimmed gradually.

The dimming time may be altered with P1 within the time range determined by network R2-C2. The circuit operates correctly only, of course, when the LDR is not exposed to light other than that from the LED. The type of LDR is not particularly important, as long as it is not too long: in the prototype, a model with a length of 5 mm was used.

## Tuesday, February 15, 2011

### Simple Remote Control Circuit

This Infrared Remote Control Software project based on Microchip 16C57 microcontroller is a reference guide to decode infrared remote control signals fromtelevision, VCR, air conditioner or other home appliances handset that uses NEC 6121 infrared format. Once one is ableto understand how to decode an IR signal of a certain format, decoding another format can be easily done as the flow chartis more or less the same except the timing of the new format.

The NEC 6121 format is based on pulse width timing in determining whether the data transmitted is "1" or "0". The data "1" is determined by the pulse width timing from one risingedge to the next rising edge of 2.24ms. The data "0" is determined by the pulse width timing from one rising edge to thenext rising edge of 1.12ms.

Most of the transmitter are modulated using a frequency of 32.75 kHz, 35.0 kHz, 36.0 kHz, 36.7 kHz, 38 kHz, 39 kHz, 40 kHz, 41.7 kHz, 48 kHz, and 56.8 kHz. The ones that are commonly used are 38 kHz and 40 kHz. In order to decode the received signals, the corresponding demodulating receiver must be used. For instance, if a modulating frequency at the transmitterused is 40 kHz, then the receiver demodulating frequency used should be 40 kHz as well. Modulating the data is a betterdesign as this will make the data integrity better and less susceptible to noise. The demodulating receivers can be obtainedfrom suppliers such as Vishay, LiteOn, Sharp or Kodenshi.

One word of caution when using the IR remote control is that it is easily affected by lighting devices that emits the infrared frequency. One such example is the fluorescent tube which emits the infrared frequency in its operation. When thistype of lights is operating, the receiver may not be able to receive the signal from the transmtter due to interference fromthe signals emitted by the flurescent tube. In situation like this, confirm this by switching off the lights when controlling the device.

You may want to consider using RF frequency as a solution in this particular location. Another way isto place a filter in front of the receiver to narrow the infrared window but this solution will compromise the angle andoperating distance of the infrared transmitter.

The infrared remote control software project provides the flow chart and source code and can be downloaded from Microchip website.

## Saturday, February 12, 2011

### Temperature Meter

The use of temperature devices in temperature measurement and sensing have made tremendous progress in the last few decades. There are a few types of measurement solutions that you can implement in your projects. The use of thermistors or thermocouples are the two most widely used devices in measurement solutions. The recent decade has seen the use of integrated circuits devices in many temperature control related systems because they are much smaller, provide a more accurate measurement and simpler to be integrated to other digital control devices.

Most of the digital temperature sensor system has a built-in communication bus to enable it to communicate with the master control IC. The most used communication interface is called I2C, a simple bi-directional 2-wire bus that was developed by Philips Semiconductors in the 1980's. Since then, many devices has this built in communication protocol that enables all devices that have this feature to be linked together without any other additional components. The I2C interfacing standard has become a world standard that are used in more than 1,000 integrated circuits.

The I2C standard basically define the start, stop, device selection addressing and data transfer interfacing protocol. The hardware consists of 2 I/O lines called SDA and SCL lines.

START Condition

The Start Data Transfer is initiated when there is a change of state of SDA line from HIGH logic to LOW logic while the SCL line is at HIGH logic. This is the START condition.
STOP Condition

The Stop Data Transfer is initiated when there is a change of state of SDA line from LOW logic to HIGH logic while the SCLline is at HIGH logic. This is the STOP condition.

DATA Transfer Condition

The data transfer is done between the START and STOP conditions with the data being transferred when SCL transition fromLOW to HIGH logic. Data is read when SCL is at HIGH logic. SDA line data will only change when SCL line is at LOW logic.There is no limit to the number of data bytes transferred and is determined by the master device. Acknowledgement of successful transfer of data is done between the master and the slave devices at regular interval.

Digital Temperature Sensor Applications

If you are into designing of thermostat controls for various buildings, industrial controls or home appliances, you maywant to consider using the TMP100 digital temperature sensor from Texas Instruments. This device can be connected tothe microcontroller using the SCL and SDA lines.

The features of the TMP100 sensor include:

* Low Quiescent standby current of 0.1uA means if you choose a proper microcontroller, the device using battery powered could last for years compared to the use of thermistor.

* Temperature range from -55 °C to 125 °C.
* Wide Power supply range from 2.7V to 5.5V.
* Accuracy of +/- 2.0 °C.
* Resolution up to 0.0625 ° C.

The typical application of the TMP100 digital temperature sensor is as shown in the diagram below.