Circuit Schematic learn

Have you ever connected two PCs together via modems using a twisted pair cable and nothing happened? That’s because the modems are expecting a phone line with all the signals and voltages supplied by the local telephone exchange. This circuit simulates the DC power and signal isolation but not the "dial tone" or the "ring signal". It suffices to connect two PCs together to communicate and exchange files using HyperTerminal. The circuit is self-explanatory and needs only one power supply for both modem lines. Although 50V DC is the usual exchange line voltage, this circuit should operate down to 20V. A 600O line transformer (eg. Jaycar cat. MM-1900) provides signal isolation, while the resistors provide current limiting and keep the lines as balanced as possible.

When using this set-up with Hyper Terminal, you should not select a Windows modem driver in the "Connect To" dialog. Instead, connect directly to the relevant COM port. Next, verify that the modems are working by sending information commands such as "ATI1" or "ATI3". If you don’t get a response using these commands, try resetting the modem(s) using the "AT&Z" command. Assuming you do get a response, set one in originate mode using the "ATD" command and the other in answer mode with the "ATA" command. If all is well, you should now be able to type in one terminal window and see the results echoed in the second PC’s terminal window. To return to control mode, type "+++". The advantage of using modems instead of a serial cable between COM ports is that the two PCs can be kilometres apart instead of a few metres. For example, you could connect the house PC to the workshop PC on the other side of the farm.

It sometimes comes as a bit of a shock the first time you need to replace the batteries in an LED torch and find that they are not the usual supermarket grade alkaline batteries but in fact expensive Lithium cells. The torch may have been a give away at an advertising promo but now you discover that the cost of a replacement battery is more than the torch is worth. Before you consign the torch to the waste bin take a look at this circuit. It uses a classic two-transistor astable multivibrator configuration to drive the LEDs via a transformer from two standard 1.5 V alkaline batteries. The operating principle of the multivibrator has been well documented and with the components specified here it produces a square wave output with a frequency of around 800 Hz.

This signal is used to drive a small transformer with its output across two LEDs connected in series. Conrad Electronics supplied the transformer used in the original circuit. The windings have a 1:5 ratio. The complete specification is available on the (German) company website at www.conrad.de part no. 516236. It isn’t essential to use the same transformer so any similar model with the same specification will be acceptable.

The LEDs are driven by an alternating voltage and they will only conduct in the half of the waveform when they are forward biased. Try reversing both LEDs to see if they light more brightly. Make sure that the transformer is fitted correctly; use an ohmmeter to check the resistance of the primary and secondary windings if you are unsure which is which. The load impedance for the left hand transistor is formed by L in series with the 1N4002 diode.

Circuit Diagram :

Two-Cell LED Torch Circuit Diagram

The inductance of L isn’t critical and can be reduced to 3.3 mH if necessary. The impedance of the transformer secondary winding ensures that a resistor is not required in series with the LEDs.Unlike filament type light sources, white LEDs are manufactured with a built-in reflector that directs the light forward so an additional external reflector or lens glass is not required.

The LEDs can be mounted so that both beams point at the same spot or they can be angled to give a wider area of illumination depending on your needs. Current consumption of the circuit is approximately 50 mA and the design is even capable of producing a useful light output when the battery voltage has fallen to 1 V. The circuit can be powered either by two AAA or AA size alkaline cells connected in series or alternatively with two rechargeable NiMH cells.

Author :  Wolfgang Zeiller Copyright : Elektor

Remote temperature measurements have to be linked by some sort of cable to the relevant test instrument. Normally, this is a three-core cable: one core for the signal and the other two for the supply lines. If the link is required to be a two-core cable, one of the supply lines and the signal line have to be combined. This is possible with, for instance, temperature sensors LM334 and LM335. However, these devices provide an output that is directly proportional to absolute temperature and this is not always a practical proposition. If an output signal that is directly proportional to the Celsius temperature scale is desired, the present circuit, which uses a Type LM45 sensor, offers a good solution.

The LM45 sensor is powered by an alternating voltage, while its output is a direct voltage. The supply to the sensor is provided by a sine-wave generator, based on A1 and A2 (see diagram). The alternating voltage is applied to the signal line in the two-core cable via coupling capacitor C6. The sensor contains a voltage-doubling rectifier formed by D1-D2-C1-C2. This network converts the applied alternating voltage into a direct voltage. Resistor R2 isolates the output from the load capacitance, while choke L1 couples the output signal of the sensor to the signal line in the cable.

Choke L1 and capacitor C2 protect the output against the alternating voltage present on the line. At the other end of the link, network R3-L2-C4 forms a low-pass section that prevents the alternating supply voltage from combining with the sensor output. Capacitor C5 prevents a direct current through R3, since this would attenuate the temperature-dependent voltage. The output load should have a high resistance, some 100 kΩ or even higher. The circuit draws a current of a few mA.