Isolated 1-Hz Clock Circuit

One of the author’s physics projects required an accurate 1-Hz (seconds) clock signal. Unfortunately, precision 10-MHz quartz crystals are expensive, while another problem was found in the inability of most common or garden 40xx CMOS logic chips to work at such a high frequency. However, a typical CMOS counter like the 4017 has such a high input resistance that its clock input has ‘radio’ properties.
Circuit diagram :
The effect is exploited here to convert the stray magnetic field picked up from a mains transformer into a clock signal. Here, the signal is induced in a short piece of wire (approx. 5 cm) connected to the clock input of a CD4017 decade counter for division by 10. The resulting 5-Hz signal is then divided by 5 by a second 4017 (IC2) to give an output of 1 Hz. LED D1 flashes to indicate the presence of a sufficiently strong magnetic field. The pickup wire should be placed close to the mains transformer, without compromising electrical safety. Always use the greatest distance at which a clock signal is reliably generated. For 1-Hz output from 60-Hz power systems, use output 6 of IC2 (pin 5).
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Condenser Mic Audio Amplifier Circuit Diagram

The compact, low-cost condenser mic audio amplifier described here provides good-quality audio of 0.5 watts at 4.5 volts. It can be used as part of intercoms, walkie-talkies, low-power transmitters, and packet radio receivers. Transistors T1 and T2 form the mic preamplifier. Resistor R1 provides the necessary bias for the condenser mic while preset VR1 functions as gain control for varying its gain. In order to increase the audio power, the low-level audio output from the preamplifier stage is coupled via coupling capacitor C7 to the audio power amplifier built around BEL1895 IC.BEL1895 is a monolithic audio power amplifier IC designed specifically for sensitive AM radio applications that delivers 1 watt into 4 ohms at 6V power supply voltage. It exhibits low distortion and noise and operates over 3V-9V supply voltage, which makes it ideal for battery operation. A turn-on pop reduction circuit prevents thud when the power supply is switched on. Coupling capacitor C7 determines low-frequency response of the amplifier. Capacitor C9 acts as the ripple-rejection filter.
Circuit Diagram :
Condenser Mic Audio Amplifier Circuit Diagram

Capacitor C13 couples the output available at pin 1 to the loudspeaker. R15-C13 combination acts as the damping circuit for output oscillations. Capacitor C12 provides the boot strapping function. This circuit is suitable for low-power HAM radio transmitters to supply the necessary audio power for modulation. With simple modifications it can also be used in intercom circuits.
Author: D. Prabakaran - Copyright: Electronics For You Mag
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Low-Cost Dual Digital Dice

This simple dual digital dice is based on three low-cost ICs, a few transistors and a handful of LEDs. IC1a & IC1b operate as an oscillator with a frequency of about 4kHz and this clocks IC2. The frequency of oscillation is not critical - it simply needs to be high enough to prevent cheating. IC2 and IC3 are 4516 binary counters, configured to count in binary from 1-6. A power-on reset is not required here since, if the initial state is outside the correct range, the counters will count into the correct range after a few clock pulses. Let's first consider how IC2 operates. When the counter reaches "7" (ie, 111), the AND gate formed by diodes D1 & D2 and the 47kO resistor applies a high to the PE pin (pin 1).
Circuit diagram:

This presets the counter to 1 (ie, 001) and so PE goes low again. The counter then increments in the normal manner until it reaches "7" again. Counter IC3 operates in the same manner except that the clock signal is derived from IC2's O3 output. The counter outputs (O1, O2 & O3) drive NPN transistors Q1-Q6 and these in turn drive the LEDs (ie, the LEDs indicate the states of the counters). Normally, the counters are incrementing continuously and the LEDs all appear to be lit. However, when push-button switch S1 is pressed, pin 6 of IC1c goes low and pin 9 of IC1d pulls the Ci input of IC2 high, thus stopping the counters. Finally, toggle switch S2 allows the user to choose between having two dice operating simultaneously or just one.
Author: Len Cox - Copyright: Silicon Chip Electronics
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Infrared Remote Extender Circuit

This project sprang from the need to be able to remotely control audio-visual equipment placed inside cupboards. RF-based commercial units such as those used for home theatre were found to be overkill for this application. The circuit is based on a commonly available infrared receiver module (IRX1) and a PIC12F675 microcontroller (IC1) – see circuit. Most infrared standards specify a nominal 38kHz carrier signal for data transmission, which the module receives and demodulates.
Circuit diagram:
infrared-remote-extender-circuit-diagram1
Digital data output:
The digital data output from the module is fed into GP2 (pin 5) of the PIC micro, where it’s received by the PIC program and duplicated on output GP1 (pin 6). This flashes the "Signal" LED to give a visual indication that the extender is receiving the remote control’s transmissions.
Copyright: Silicon Chip
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Power Flip-Flop Using A Triac

Modern electronics is indispensable for every large model railroad system, and it provides a solution to almost every problem. Although ready-made products are exorbitantly expensive, clever electronics hobbyists try to use a minimum number of components to achieve optimum results together with low costs. This approach can be demonstrated using the rather unusual semiconductor power flip-flop described here. A flip-flop is a toggling circuit with two stable switching states (bistable multivibrator). It maintains its output state even in the absence of an input pulse.
Flip-flops can easily be implemented using triacs if no DC voltage is available. Triacs are also so inexpensive that they are often used by model railway builders as semiconductor power switches. The decisive advantage of triacs is that they are bi-directional, which means they can be triggered during both the positive and the negative half-cycle by applying an AC voltage to the gate electrode (G). The polarity of the trigger voltage is thus irrelevant. Triggering with a DC current is also possible. Figure 1 shows the circuit diagram of such a power flop-flop. A permanent magnet is fitted to the model train, and when it travels from left to right, the magnet switches the flip-flop on and off via reed switches S1 and S2.
Circuit diagram:
Power Flip-Flop Using A Triac

In order for this to work in both directions of travel, another pair of reed switches (S3 and S4) is connected in parallel with S1 and S2. Briefly closing S1 or S3 triggers the triac. The RC network C1/R2, which acts as a phase shifter, maintains the trigger current. The current through R2, C1 and the gate electrode (G) reaches its maximum value when the voltage across the load passes through zero. This causes the triac to be triggered anew for each half-cycle, even though no pulse is present at the gate. It remains triggered until S2 or S4 is closed, which causes it to return to the blocking state.The load can be incandescent lamps in the station area (platform lighting) or a solenoid-operated device, such as a crossing gate. The LED connected across the output (with a rectifier diode) indicates the state of the flip-flop.
The circuit shown here is designed for use in a model railway system, but there is no reason why it could not be used for other applications. The reed switches can also be replaced by normal pushbutton switches. For the commonly used TIC206D triac, which has a maximum current rating of 4 A, no heat sink is necessary in this application unless a load current exceeding 1 A must be supplied continuously or for an extended period of time. If the switch-on or switch-off pulse proves to be inadequate, the value of electrolytic capacitor C1 must be increased slightly.
Author: R. Edlinger - Copyright: Elektor July-August 2004
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Portable 9v Headphone Amplifier

High Quality One-IC unit, Low current consumption
After several requests by correspondents, the decision of designing a 9V powered Headphone Amplifier was finally taken. The main requirement was to power the circuit by means of a common, PP3 (transistor radio) alkaline battery. So, implementing a low current drawing circuit was absolutely necessary, though preserving a High Quality performance.

Circuit Diagram:
Portable 9v Headphone Amplifier Circuit Diagram
Parts:
P1 = 22K
R1 = 18K
R2 = 68K
R3 = 68K
R4 = 68K
R5 = 18K
R6 = 68K
C1 = 4.7uF-25v
C2 = 4.7uF-25v
C3 = 22pF
C4 = 220uF-25v
C5 = 220uF-25v
C6 = 4.7uF-25v
C7 = 22pF
C8 = 220uF-25v
J1 = 3.5mm Stereo Jack
B1 = 9V Alkaline Battery
IC1 = NE5532-34
SW1 = SPST Toggle Switch

The appearance of the 5534 low-noise op-amp at a reasonable price was much appreciated by audio designers. It is now difficult or impossible to design a discrete stage that has the performance of the 5534 without quite unacceptable complexity. 5534 op-amps are now available from several sources, in a conventional 8-pin d.i.l. format. This version is internally compensated for gains of three or more, but requires a small external capacitor (5-15pF) for unity-gain stability. The 5532 is a very convenient package of two 5534s in one 8-pin device with internal unity-gain compensation, as there are no spare pins.

The 5534/2 is a low-distortion, low-noise device, having also the ability to drive low-impedance loads to a full voltage swing while maintaining low distortion. Furthermore, it is fully output short-circuit proof. Therefore, this circuit was implemented with a single 5532 chip forming a pair of stereo, inverting amplifiers, having an ac gain of about 3.5 and capable of delivering up to 3.6V peak-to-peak into a 32 Ohm load (corresponding to 50mW RMS) at less than 0.025% total harmonic distortion (1kHz & 10kHz). If we consider that the mean current drawing at a power output of 15mW per channel is around 12-13mA (both channels driven), this Headphone Amplifier will become a 'must' for many DIY enthusiasts needing a High Quality, High Performance portable device.

Technical data
Sensitivity:
    200mV RMS for 15.6mW RMS output
    350mV RMS for 50mW RMS output
Maximum undistorted output: 3.6V Peak-to-peak
Frequency response: flat from 40Hz to 20KHz; -2.3dB @ 20Hz
Total harmonic distortion @ 1KHz: <0.025% at all power outputs up to 50mW RMS
Total harmonic distortion @10KHz: <0.02% at all power outputs up to 50mW RMS
Total current drawing @ 9V supply (both channels driven):
    Standing current: 8.5mA
    Mean current drawing @ 15mW RMS per channel: 12mA
    Mean current drawing @ 35mW RMS per channel: 17mA
 Source : www.redcircuits.com
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Temperature Sensitive Switch For Solar Collector

This circuit can be used to turn the pump on and off when a solar collector is used to heat a swimming pool, for example. This way the water in the collector has a chance to warm up significantly before it is pumped to the swimming pool. A bonus is that the pump doesn’t need to be on continuously. The basis of operation is as follows. When the temperature of the water in the solar collector is at least 10 °C higher than that of the swimming pool, the pump starts up. The warm water will then be pumped to the swimming pool and the temperature difference will drop rapidly. This is because fresh, cool water from the swimming pool enters the collector. Once the difference is less than 3 °C the pump is turned off again. R10/R1 and R9/R2 each make up a potential divider. The output voltage will be about half the supply voltage at a temperature around 25 °C. C7 and C8 suppress any possible interference. The NTCs (R9 and R10) are usually connected via several meters of cable, which can easily pick up interference. Both potential dividers are followed by a buffer stage (IC1a/IC1b). IC1c and R3, R4, R5 and R6 make up a differential amplifier (with unit gain), which measures the temperature difference (i.e. voltage difference).
When both temperatures are equal the output is 0 V. When the temperature of the solar collector rises, the differential amplifier outputs a positive voltage. This signal is used to trigger a comparator, which is built round an LM393 (IC2a). R7 and P1 are used to set the reference voltage at which the comparator changes state. R8 and P2 provide an adjustable hysteresis. R11 has been added to the output of IC2a because the opamp has an open collector output. A power switch for the pump is created by R12, T1 and Re1. D1 protects T1 against voltage spikes from the relay coil when it is turned off. A visual indication of the state of the controller is provided by IC4 (UAA170), a LED spot display driver with 16 LEDs. The reference voltage for the comparator is buffered by IC1d and fed to input VRMAX of the UAA170. R20/D21 and R23/D22 limit the input voltages of IC4 to 5.1 V, since the maximum permissible input voltage to the UAA170 is 6 V. When there is no temperature difference, LED D20 turns on.
Circuit diagram:
Temperature Sensitive Switch For Solar Collector
As the temperature difference increases the next LED turns on. The full scale of the LED bar is equal to the reference voltage of the comparator. This means that when the last LED (D5) of the UAA170 turns on, the comparator switches state. This is also indicated by D2. The power supply has been kept fairly simple and is built around a LM7812 regulator. The circuit is protected against a reverse polarity at the input by D3. You have to make sure that the input to the regulator is at least 15 V, otherwise it won’t function properly. There are a few points you should note regarding the mounting of the NTCs. NTC R9 should be placed near the output of the solar collector. You should choose a point that always contains water, even when some of the water flows back a little. NTC R10 should be mounted inside the filter compartment (where it exists), which continually pumps the swimming pool water.
This will give a good indication of the temperature of the water. The way the circuit has to be set up depends how it has been installed and is very much an experimental process. To start with, set hysteresis potentiometer (P2) halfway. Then set the reference voltage to about 1.5-2 V with P1. On a sunny day you can measure the voltage difference to get an idea as to which reference voltage needs to be adjusted. The hysteresis setting determines how long the pump stays on for, which is until the minimum temperature difference has been reached.
Author: Tom Henskens - Copyright: Elektor Electronics
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