Circuit knowledge

I truly do dislike virtually any circuit hooked up straight to 240v mains. Nevertheless we find Christmas time tress illumination being connected straight away to the mains always since years on end having no major disorders.

Isolation need to be supplied and the lights (LEDs) needs to be off from prying hands.
You will need a minimum of 50 LEDs in every single thread to stop them being harmed through a power upswing through the 1k resistor - in case the circuit is started up at the highest of the waveform. When you increase LEDs to each line, the current will reduce a very small quantity right up till, when you have 90 LEDs in each thread, the current will be nil.
For 50 LEDs in each thread, the whole feature voltage will be 180v to ensure that the peak voltage will be 330v - 180v = 150v. Each LED will find a lot less than 7mA highest while in the half-cycle they may be switched on. The 1k resistor may possibly cancel 7v - due to the fact the RMS current is 7mA (7mA x 1,000 ohms = 7v). Simply no rectifier diodes are needed. The LEDs are the "rectifiers." Really clever. You should have LEDs in both directions to charge and discharge the capacitor. The resistor is furnished to obtain a heavy surge current from one of the series of LEDs if the circuit is triggered whenever the mains is at a optimum.
This is often up to 330mA in the event only 1 LED is employed, therefore the benefit of this resistor needs to be modified as long as a couple of LEDs run. The LEDs above pick up apex current.
A 100n cap generates 7mA RMS or 10mA apex in full wave or 3.5mA RMS (10mA peak for one half a cycle) in half-wave. (when solely 1 LED remains in each sequence).

A gas sensor circuit discussed in the previous article can be also implemented for testing whether a vehicle driver is drunk to dangerous levels or not. Here we learn the construction of this circuit and some other important hints regarding the circuit. A comprehensive information regarding the assembly of the proposed alcohol tester circuit can be seen in this article. The sensing tube is made from a piece of polythene tubing, about 25 cm long with an inside diameter of about 15 mm, into which the gas sensor fits tightly (take the sensor with you when you go to buy the tubing). The tubing should be securely glued to the inside and outside of the case wall with two-component epoxy resin.

Fit the gas sensor in the foremost part of the tube: if the diameter of the tube is slightly too large, wind some tin or aluminium foil around the sensor until the whole fits tightly. Under no circumstances should you use any glue near the sensor as this could give rise to spurious indications for a long time to come. The sensor should therefore be secured with a suitable aluminium or plastic hood as shown in the figure. The openings behind the sensor in the tube ensure that the sensor is well immersed in exhaust gas so that it works fairly rapidly. Furthermore, these openings prevent a build-up of gas behind the sensor (which is pervious to gases) that could give rise to spurious readings. The LEDs should be mounted in their sockets in the top moulding of the case. Spray a liberal quantity of lacquer over their terminals, or, better still, brush some two- component epoxy resin over them, which will make the whole sturdier in use.

Calibration: Operate the tester in the open air and, using P1, set the voltage on pin 8 of IC2 to a level between 1 and 3 V. If that is not possible, reduce the value of P1. Next let the gas sensor heat up for at least a couple of hours. The setting of the thresholds of the window discriminator to correspond to 1 per cent and 4.5 per cent CO content requires the use of a reference tester. Many garages, MOT testing stations, as well as the test centres of the AA and RAC have such a unit, and you should be able (during a quiet period) to get the use of it for a little while. During the measurement hold the preheated gas sensor in the gas stream about 10 cm from the exhaust outlet. After a short while, the sensor has stabilized and can then be compared with the reference. Adjust the choke (engine running!) so that the reference meter indicates a CO content of 4.5 per cent.

On the CO tester set P2 such that the red LED just does not light: if you really love the environment, set P2 so that the red LED just lights. Follow the same procedure with a CO content of 1 per cent (hopefully this is possible), but in this case adjust P3 until the yellow LED just lights. As already stated before, the accuracy of measurement is set between narrow limits; none the less, the CO tester makes it possible to check the cleanliness of your engine regularly and conveniently. It is possible to power the tester from the cigarette lighter via a suitable two-core cable (about 3 m long): a plug for this purpose is obtainable from most good car accessory shops.

the proposed alcohol gas sensor proper is based on a mixture of chemical and physical properties. Basically, when heated, it behaves like a resistance of which the value diminishes when the CO content increases. Perhaps that is putting it a little too simple, because the sensor reacts not only to carbon monoxide, but also to a number of other gases and gaseous substances in the atmosphere, from alcohol, which is blue but indicated by the red LED, through propane and butane to the carbon hydroxides (found, for instance, in stink bombs). This is illustrated in figure below. None the less, the primary harmful constituent of the exhaust gases is carbon monoxide which is detected with great accuracy. Flashing of the LEDs indicates that the tester is not yet ready for use, because the gas sensor has not heated up. Only when the LEDs stop flashing, that is, after about 3. . .5 minutes, can the measurements be commenced.

 Construction hints for the alcohol tester circuit

First, the voltage regulator, IC1. This integrated device provides a stabilized supply of 5 V at a maximum current of 1 A from the pulsating battery voltage. Capacitors C1 and C2 provide further smoothing of the supply voltage. The heating element of the gas sensor is connected in parallel with the regulator. Both IC1 and the sensor get noticeably warm during use and IC1 is therefore fitted onto a heat sink which improves heat conduction in the closed case. The regulator also provides thresholds for comparator lC2 which are preset by P2 and P3.

This circuit is used for optimum regulation of the flow of hot water in a central heating system.
It measures the water temperature, and arranges for a particular valve or pump in the system to be switched on to achieve a user-defined temperature distribution in the home. Residual heat in the central heating system can thus be used to lower the cost of fuel. Fig. 1 shows that water in temperature range I can be used for the central heating and the storage vessel, while that in range II is also suitable for directing to the boiler. In most cases, it is not recommended to re-use water with a temperature below 30 °C. The circuit arranges for an alarm to be activated when the water temperature falls below 5°C, or exceeds 95 °C. The circuit diagram of the central heating control appears in Fig. 2. Relays Re1 and Res are activated upon measuring the maximum and minimum permissible temperature, respect- ively.

The temperature sensor is a Type LM35, which has a scale factor of +10 mV/°C. Its output voltage is amplified in At and fed to the non-inverting inputs of comparators A2-A6. The presets at the inverting input of each of these is used to set the toggle voltage, i.e., the temperature at which the relevant relay is switched on or off. The relay drivers are open-collector power buffers with built-in freewheeling diodes to afford protection against inductive surges.

The use of the Type ULN2003 makes it possible to use relays with a coil voltage of upto 50 V without the need for additional interfacing. Each temperature setting has a hysteresis of about 2 °C. Transistors T1-Ta serve to disable the previously energized pump or valve upon detecting a water temperature that falls within another, predefined, range. In this manner, only one relay is activated at a time. It stand to reason that the temperature sensor, IC1, must be mounted such that it is in thermal contact with the water in the heating system. Make sure that the device is well-insulated, and that it does not cause leakage. The temperature range settings for the presets are shown opposite.

This affordable FM radio receiver antenna enhancer makes use of the BF324 TO92 type pnp transistor in a grounded-base arrangement.
The circuit can be utilized like a signal booster with VHF receivers whose front end is afflicted with reduced level of sensitivity (like a lot of valved and army supplemental variations). The frequency selection of the preamplifier is approximately from 75MHz to 150MHz.

The 2 inductors in the circuit are do-it-yourself. L1 includes 10 turns of 24SWG enameled copper wire; the inner dimension is 3mm, without any core. Inductor L2 bears 13 flips of the identical wire, as well as an inner diameter of 5mm; without core as above. A building tip: tight-wind the inductors making use of three and 5mm drill bits respectively in the form of momentary cores.

The prototype of the preamplifier was effectively tried with an 88-108 MHz FM broadcast receiver along with a 2-metre VHF ham receiver. The preamplifier consumes around 2.5mA from a 5-volt supply.

To maintain oscillation the op. amp. must have a gain equal or exceeding this attentuation which is in fact x3. The desired gain is obtained by selecting the ratio of feedback resistance to input resistance of the inverting input (RV2 + R3)/R2.
lf the overall gain, including feedback, exceeds unity the circuit will produce sine wave oscillation at a frequency set by the Wien network. Stabilisation of the gain is brought about by the action of diodes D1 and Dl% then  the instantaneous output voltage is close to zero, neither diode conducts, since even a germanium diode requires 0.4 volts or so forward voltage to bias it on. Consequently, the negative feedback loop is open (giving maximum gain) and, under the action of the positive feedback via the Wien network, oscillations build up rapidly. As soon as their amplitude is  sufficient to bias on either Dl or D2 (depending on the polarity of the output voltage swing), then R2, R3 and RV2 provide negative feedback, so limiting oscillations to a convenient level. Reinforcement of such oscillations takes place close to each zero crossing when D1 and D2 are open ire. non·conducting; the setting of RV2 determines the final amplitude.

This method of stabilisation does give rise to a very small amount of crossover distortion, but the effect of this can be minimised by setting VR2 for the largest possible sine wave without clipping. ln any event, some distortion is a small price to pay for such a simple, easy-to~get—working sine wave oscillator and, further, it is a low level of distortion — some class B audio amplifiers are worse! Range switching is confined to a choice of two ranges, in the interest of simplicity and cheapness, but more ranges could easily be provided if the  constructor is so inclined. A simple emittentouower output stage completes the unit, with a logarithmic potentiometer as a level control, enabling the output to be set from 1 V rms down to 10 mV rms or so. With wiring up completed and thoroughly checked, switch on and, if possible, monitor the output on an oscilloscope. No ’scope? Then a pair of headphones, of reasonably high impedance, can be used instead. Set RV4 about half way, S1 to "low" and RVI about half way. lf there is no output, adjust RV2; clockwise rotation should give increased output. With an ac meter, measure the signal level at the junction of D1, D2 and RV2. Adjust RV2 for 3 volts rms. This will ensure the highest output level (thus reducing the effect of crossover distortion) consistent with sine wave operation (no`clipping). This should provide about one volt rms at the output. 

By replacing the common-emitter resistor in a conventional Schmitt by a zener diode,the hysteresis normally associated with these circuits is eliminated. 



One cheap IC, ul.914, can be used as an extremely simple and effective Schmitt trigger suitable for many applications. Hysterisis of the circuit is about 0.1 Volt. This may be varied by altering the values of R1 and R2. 



An LED and phototransistor optical coupler system may be used to lengthen a three microsecond pulse to 55 milliseconds by using the values shown in the diagram. This allows complete isolation between input pulse circuits and the lengthener to be obtained. 

The generator described is a good value·for money alternative to the commercially available types; it uses  only one CMOS-IC 4060 and provides up to seven different baud rates. Lets learn more regarding the pin out configuration of the IC 4060.

Each serial data interface requires a baud rate generator: terminal, printer tape interface. . . lt is often not possible to use the same generator for input and output, so it would be useful to add one to, for instance, the UART in the computer or the UART in the terminal. To keep the ,costs of such an addition down, we have developed a generator which uses just one IC, two resistors, one preset potentiometer and a capacitor. The CMOS 4060 is a 14-stage binary counter with an internal oscillator which only requires the frequency determining RC components. As the reset is connected to earth, the counter begins to count upwards when the supply is switched on. A clock frequency then becomes available at the outputs: the higher the number of the output, the lower the frequency available at it.

 In the circuit shown the frequencies avail- able at the various outputs are: O4 = 9600 baud 08 = 600 baud 05 = 4800 baud O9 = 300 baud O6 = 2400 baud Q10 = 150 baud Q7 = 1200 baud If the outputs are wired as shown in the circuit diagram, the required baud rate can then be selected by means of a wire link. _ The oscillator frequency can be set precisely with P1, and measured either at pin 9 of the 4060 or at one of the outputs O4 . . . Q10. With the values shown, the frequency at pin 9 should be 38.4 kl-lz; at the outputs O4. . .010 the relevant baud rate. lt is often required that the clock frequency is 16 times the baud rate (for instance, with asynchronous ~ operation of the 6850, 8251, 280- SIO. . .). ln that case, C1 must be replaced by a 27 nF capacitor and the oscillator frequency must be set to 614.4 kHz.

The theoretical circuit is given in Fig. 1 and consists of a Colpitt’s oscillator using 8MHz crystals. Six channels are shown in the schematics—three, in fact, `T were used for the prototype but there is no reason why many crystals cannot be included by using a suitable multiway switch and increasing the number of islands on the board; using the smaller HC25 series crystals would permit more channels to be fitted in the space allotted. The trimmers in series with each crystal allow easy netting to the assigned frequency. The f.m. is applied to the oscillator by a reactance stage, fed by two audio pre-amps. Deviation is con- trolled by a 10kQ potentiometer and the maximum attained on the prototype was 8kHz. Notice the inclusion of decoupling in the audio stages to prevent r.f. pickup so often a cause of poor audio quality in home·constructed equipment. The printed board layout is shown in Fig. 2.

Making PCBs

 Cut a piece of single-sided copper board to the size shown and with some {ine abrasive paper, clean the copper surface to remove any oxide or tarnish.

Using a soft, lead pencil, draw out the islands on the board, and then draw around these and the inter- connections of the earth plane edge. The small islands and fine connections are then filled in by means of an etch-resist pen or ine paint brush, using quick drying paint, such as car touch-up paint, thinned down if necessary. The larger areas are then put in care- fully and when the board is dry, each island and connection examined to make sure no copper bridges exist between them. One should also ensure adequate clearances.

Place the board in a suitable plastic or earthen- ware container and pour on just sufficient ferric chloride solution as is necessary to cover it. The solution can be purchased ready-mixed from most radio component stores, or can be made up by a chemist. It is however a corrosive, albeit a mild one, so handle carefully and wash off any of the solution that comes into contact with the skin immediately.

 Initially, leave the board submerged for about twenty minutes, agitating occasionally. You will see the chemical action taking place quite clearly and when all the unwanted copper has been eroded, take out the p.c.b., wash in clean water and then dry. Using a wet abrasive pad such as a Brillo pad- the paint is now removed and a final wash and dry will leave the copper gleaming. After a final check of the work, drill the mounting holes for fixing to the metal chassis.

Each board in the transmitter is etched in this way and provided the simple instructions are followed you should easily be able to provide good examples.

Mounting Components

There is no hard-and-fast rule about fixing the components to the board, but the Author favours soldering the resistors first, followed by the capacitors, the coils and finally the transistors. Keep lead lengths  short typically 6-12mm for transistors and solder neatly, holding the iron in place just long enough for the solder to flow to the joint. An iron of 15W rating with a bit size of 3mm or so is to be preferred for work of this nature.

Testing the long range transmitter circuit

Connect a 15 volt supply to the board having first established that the polarity is correct and check the voltages shown: a 15 per cent error is quite acceptable, due to component tolerances. With a 6009 microphone and a pair of earphones across C11 to the earth line, check for clean audio and the operation of the deviation control. The oscillator can be tested by connecting a suitable 8MHz crystal in position (i.e. 8-08335MHz for S20-145-5MHz) and listening for the 8MHz signal on a tunable h.f. receiver, coupled loosely to the vicinity of the oscillator stage.

After this feed an audio signal at the input and try to receive this over any standard 2 mtere band receiver unit placed at about 10 meters distance.

With little trial and error you would be able to receive a crystal reception, done! Now you can take the unit to some other far away location and confirm the same.