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Monday, October 4, 2010

Smart Heater Controller

Minuscule circuit of the electronic heater controller presented here is built around the renowned 3-Pin Integrated Temperature Sensor LM35 (IC1) from NSC. Besides, a popular BiMos Op-amp CA3140 (IC2) is used to sense the status of the temperature sensor IC1, which also controls a solid-state switch formed by a high power Triac BT136(T1). Resistive type electric heater at the output of T1 turns to ON and to OFF states as instructed by the control circuit. This gadget can be used as an efficient and safe heater in living rooms, incubators, heavy electric/electronic instrument etc.Normally, when the temperature is below a set value (Decided by multi-turn preset pot P1), voltage at the inverting input (pin2) of IC1 is lower than the level at the non-inverting terminal (pin3). So, the comparator output (at pin 6) of IC1 goes high and T1 is triggered to supply mains power to the desired heater element.

When the temperature increases above the set value, say 50-60 degree centigrade, the inverting pin of IC1 also goes above the non-inverting pin and hence the comparator output falls. This stops triggering of T1 preventing the mains supply from reaching the heater element. Forunately, the threshold value is user-controllable and can be set anywhere between 0 to 100 Degree centigrade.

The circuit works off stable 9Volt dc supply, which may be derived from the mains supply using a standard ac mains adaptor (100mA at 9V) or using a traditional capacitive voltage divider assembly. You can find such power circuits elsewhere in this website.

Note:CA3140 (IC2) is highly sensitive to electrostatic discharge (ESD). Please follow proper IC Handling Procedures.

How to build Opamp VHF FM Transmitter

ICs that in the past were far too expensive for the hobbyist tend to be more favourably priced these days. An example of this is the AD8099 from Analog Devices. This opamp is available for only a few pounds. The AD8099 is a very fast opamp (1600 V/ms) and has high-impedance inputs with low input capacitance. The bandwidth of the opamp is so large that at 100 MHz it still has a gain of nearly 40. This means that this opamp can be used to create an RC oscillator. The circuit presented here realises that.
The circuit has a few striking characteristics. Firstly, unlike normal oscillators that contain transistors this one does not have any inductors. Secondly, there is no need for a varicap diode to do the FM modulation. The opamp is configured as a Schmitt trigger with only a small amount of hysteresis. The output is fed back via an RC circuit. In this way, the trimmer capacitor is continually being charged and discharged when the voltage reaches the hysteresis threshold. The output continually toggles as a consequence.
This results in a square wave output voltage. With a 10-pF trimmer capacitor the frequency can be adjusted into the VHF FM broadcast band 88-108 MHz). The frequency of the oscillator is stable enough for this. The output voltage is about 6 Vpp at a power supply voltage of 9 V. The transmitter power amounts to about 50 mW at a load of 50R. This is about 20 times as much as the average oscillator with a transistor. With a short antenna of about 10 cm, the range is more than sufficient to use the circuit in the home as a test transmitter.
Because the output signal is not free from harmonics the use of an outdoor antenna is not recommended. This requires an additional filter/adapter at the output (you could use a pi-filter for this). The FM modulation is achieved by modulating the hysteresis, which influences the oscillator frequency. An audio signal of about 20 mVpp is sufficient for a reasonable output amplitude. The package for the opamp is an 8-pin SOIC (provided you use the version with he RD8 suffix). The distance between the pins on this package is 1/20 inch 1.27 mm).
This is still quite easy to solder with descent tools. If SMD parts are used for the other components as well then the circuit can be made very small. If necessary, a single transistor can be added to the circuit to act as microphone amplifier. The power supply voltage may not be higher than 12 V, because the IC cannot withstand that. The current consumption at 9 V is only 15 mA. As with all free-running oscillator circuits, the output frequency of this specimen is also sensitive to variations of the power supply voltage.
For optimum stability, a power supply voltage regulator is essential. As an additional design tip for this circuit, we show an application as VCO for, for example, a PLL circuit. When the trimmer capacitor is replaced with a varicap diode, the frequency range can be greater than that of an LC oscillator. That’s because with an LC-oscillator the range is proportional to the square root of the capacitance ratio. With an RC oscillator the range is equal to the entire capacitance ratio. For example: with a capacitance ratio of 1:9, an LC oscillator can be tuned over a range of 1:3.
With an RC oscillator this is 1:9. For the second tip, we note that the circuit can provide sufficient power to drive a diode mixer (such as a SBL-1) directly. This type of mixer requires a local oscillator signal with a power from 5 to 10 mW and as already noted, this oscillator can deliver 50 mW. A simple attenuator with a couple of resistors is sufficient in this case to adapt the two to each other.


Pulse generator with 555

This is a pulse generator with adjustable duty cycle made with the 555 timer IC. The circuit is an astable multivibrator with a 50% pulse duty  cycle. The difference from the standard design of a 555 timer is the resistance between pins 6 and 7 of the IC composed of P1, P2, R2, D1 and D2.The diodes D1 and D2 set a definite charging time for C1 which produces a 50% duty cycle in a normal case. The duty cycle (n) is dependent on P1 and P2 in the
 following manner:
n = 1 + P2/P1
If P2 = 0 (n = 100%) then the frequency can be approximately calculated with the following formula:
f = 0.69/((2*P1 + P2 + 4.7kΩ)*C1)
Components List
C2 = 10µF
C3 = 0.1µF
R1 = R2 = 4.7K
D1 = D2 = 1N4148
IC = 555
C1, P1 and P2 must be calculated

Piezoelectric Heat Sensor

Here is an Ultrasensitive Heat Sensor to monitor high temperature in Electronic devices. It can be placed inside the electronic gadget that usually generates heat during its operation. The circuit also functions as a sensitive Fire Alarm. This heat sensor uses the piezoelectric property to sense heat.
The sensor element is the ordinary piezo disc found in buzzer. The middle part of the piezo disc is coated with a layer of piezo electric material called Lead Zirconate. The crystals in this material are capable of dis-orientation and re-orientation when it is subjected to mechanical, electrical and heat stress. The Direct Piezo electric property of the piezoelectric crystals is the ability to generate electric signals when the crystals dis-orient and re-orient following a stress. This property is used here to sense temperature.

A high gain type OP-Amp is used to sense the electrical signals from the piezo disc. The inverting (pin2) and non – inverting (pin3) inputs of the Op-Amp IC CA3140 are shorted through the capacitor C1, so that both the inputs will be in a balanced state. When this happens, output of IC1 remains low. When the piezo disc is subjected to heat stress, it generates a very small current that is enough for upsetting the balance of the inputs of IC1. This changes the output of IC1 from low to high. This high output is used to activate the alarm circuit based on the ROM IC UM3561.
UM3561 is a ROM IC used to generate alarm tones like Fire engine, Ambulance, Machine gun, Police sirens etc. These sounds depend on its pin connection (pin6). Here it is used as a Fire Brigade Siren by leaving its pin6 unconnected. UM3561 is a low power IC requires 3 volts. So a Zener diode is used to reduce the 9 volt supply to 3 volts. Resistor R4 (220K) is important to maintain the oscillating frequency of the ROM IC. Since the output from IC2 is too weak, transistor T1 is used to amplify the sound for the speaker. Capacitors C2 and C3 are included as buffers so that, they will keep the voltage level for IC2 for some time even if the output of IC1 turns low. Glue the fine side of the Piezo disc on the cabinet of the equipment close to the Heat sink or similar heat generating devices. No adjustment is necessary. Power (9-12 volt DC) can be obtained from the equipment itself.