Bicycle Horn Using Tone Generator KA2411

An interesting circuit of a bicycle horn based on a popular, low cost telecom ringer chip is described here. This circuit can be powered using the bicycle dynamo supply and does not require batteries, which need to be replaced frequently. The section comprising diodes (D1 and D2) and capacitors (C1 and C2) forms a half-wave voltage-doubler circuit. The output of the voltage doubler is fed to capacitor C3 via resistor R1. The maximum DC supply that can be applied to the input terminals of IC1 is 28V. Therefore zener diode ZD1 is added to the circuit for protection and voltage regulation. The remainder of the circuit is the tone generator based on IC1 (KA2411).

Video Switch for Intercom System IC 4060

Nowadays a lot of intercom units are equipped with video cameras so that you can see as well as hear who is at the door. Unfortunately, the camera lens is perfectly placed to serve as a sort of support point for people during the conversation, with the result that there’s hardly anything left see in the video imagery. One way to solve this problem is to install two cameras on the street side instead only one, preferably some distance apart. If you display the imagery from the two cameras alternately, then at least half of the time you will be able to see what is happening in front of the door.

Circuit diagram:

click for larger image

Video Switch for Intercom System Circuit Diagram

Stereo LED Power (VU) Meter

This circuit senses AC audio voltage supplied to the car-radio loudspeakers and displays it as power using a LED bar graph, achieving at the same time an attractive visual effect. It is designed to cover common car-radio output power ranges, but can easily be modified to suit different needs. It is supplied from the 12 V car electrical system and is suitable for classical CC (Capacitor-coupled) as well as BTL (Bridge Tied Load) types of amplifier with no changes to the circuitry or connections at all. In fact, only the meaning of the LEDs changes — with BTL, the LED increments equal four times the CC value on the same load. CC-type amplifiers have the loudspeaker connected via a DC-decoupling capacitor at the output and ground (negative).

BTL-type amplifiers, on the other hand, have the loudspeaker DC-coupled and ‘stretched’ between two equal, parallel, but phase-reversed outputs. The result compared to ‘CC’ is twice the voltage swing, hence quadrupling the power being fed to the same loudspeaker load. It is necessary to know to which of the two types this circuit is connected to only in order to correctly assign power levels (W) to the LEDs. CC-type have no DC voltage to ground at the outputs and return wires. The return wires are actually connected to the common ground (negative).

Real RS232 For Laptop/Notebook PCs

Many Metex DVMs feature a serial interface (RS232) which enables measured values to be copied to a PC for processing. Although this works just fine with most desktop PCs, problems may arise — as the author found out the hard way — when a laptop PC like the IBM Thinkpad 370C is hooked up to the Metex DVM. The cause of the problems is the limited voltage swing of just ±5V on the 370C’s serial interface. This is simply not enough for the Metex DVM, which will appear ‘deaf’ to the laptop.

Microscope 2.5-D Lighting

Interesting, surprising as well as scientifically useful visual effects can be obtained by employing colored LEDs instead of the normal lamp fitted in the lighting assembly in a microscope base. Of course, the lamp can be simply replaced by a white LED with appropriate changes in the supply current/voltage etc., but it is more interesting to have three colors available — here, red, green and orange (amber) which are individually adjustable for intensity. The dashed line around LEDs D2, D3 and D4 indicates that they are fitted in a 10 mm long section of 1-inch plastic (PVC) pipe.

Two Keyboards on one PC

This circuit does the exact opposite of what most of this type of changeover switches attempt to do. Usually a changeover switch is used with two PCs and one keyboard. This version however makes it possible to operate one PC with two keyboards.

K1 is connected to the PC for this purpose while K2 and K3 are connected to two keyboards. The data outputs of the keyboards are high in the idle state. As soon as a key is pressed the keyboard will serially transmit the data. The data line will now also be low from time to time.

DRM Direct Mixer EF95/6AK5

This hybrid DRM receiver with a single valve and a single transistor features good large-signal stability. The EP95 (US equivalent: 6AK5) acts as a mixer, with the oscillator signal being injected via the screen grid. The crystal oscillator is built around a single transistor.

Converting a DCM Motor

We recently bought a train set made by a renowned company and just couldn’t resist looking inside the locomotive. Although it did have an electronic decoder, the DCM motor was already available 35 (!) years ago. It is most likely that this motor is used due to financial constraints, because Märklin (as you probably guessed) also has a modern 5-pole motor as part of its range. Incidentally, they have recently introduced a brushless model. The DCM motor used in our locomotive is still an old-fashioned 3-pole series motor with an electromagnet to provide motive power. The new 5-pole motor has a permanent magnet.

We therefore wondered if we couldn’t improve the driving characteristics if we powered the field winding separately, using a bridge rectifier and a 27 Ω current limiting resistor. This would effectively create a permanent magnet. The result was that the driving characteristics improved at lower speeds, but the initial acceleration remained the same. But a constant 0.5 A flows through the winding, which seems wasteful of the (limited) track power. A small circuit can reduce this current to less than half, making this technique more acceptable. The field winding has to be disconnected from the rest (3 wires).

Converting a DCM Motor

Incandescent Lamps Life Extender

One way to extend the operating life of incandescent lamps is to reduce the voltage across the lamp by connecting something in series with it. Naturally, a series resistor is the simplest solution, but this produces unnecessary power dissipation. In order to reduce the voltage on a 100-W lamp by approximately 10 V, slightly more than 4 W must be dissipated in the resistor, and that is naturally a waste of energy. A better approach is to use a phase control circuit with a triac controller to reduce to voltage on the lamp. To reduce the voltage on the lamp by 10 V, a phase cut of approximately 44 degrees is necessary.

Garden Lighting

Completely ‘self-supporting’ garden lamps using solar cells as their energy source are gradually becoming more and more common. How do they actually work? We took one apart to find out. From environmental and technical considerations, buying such a solar-cell garden lamp has a lot to recommend it. It’s a great thing that the energy necessary for the lamp to burn in the evening can be drawn from the sunlight that is available for free during the day. In addition, such a lamp is enormously practical, since you can place it in any desired location in the garden without having to dig a trench through the lawn or flowerbeds.

Mini Lithium Torch

This mini pocket torch combines the advantages of Lithium button cells with a super-bright white LED. The lithium cells are small, have a long shelf life and have very little self-discharge. The LED has very high efficiency, an extremely long life expectancy and a modest current consumption (20mA). In combination this results in an exceptionally long-life pocket torch. It is unfortunate that a Lithium cell has a voltage of only 3 V, while a super-LED has a forward voltage of 3.5 V. One cell will not suffice and we therefore will have to connect two in series.

Optimised Semiconductor Noise Source

We have already published designs that use a transistor junction operating in Zener breakdown as a noise source. Anyone who has experimented with a reverse-biased transistor knows that the amplitude of the noise voltage generated in this manner is strongly dependent on the supply voltage. The variation between individual transistors is also rather large. An obvious solution is to use an adjustable supply voltage for the noise generator stage. A BC547B starts to break down at around 8V.

Thermal Fan Controller, IC 741

This fan controller uses one or more ordinary silicon diodes as a sensor, and uses a cheap opamp as the amplifier. The circuit designed for 12V computer fans, as these are now very easy to get cheaply. These fans typically draw about 200mA when running, so a small power transistor will be fine as the switch. I used a BD140 (1A, 6.5W), but almost anything you have to hand will work just as well.

Circuit diagram:

 click for large image

Thermal Fan Controller Circuit Diagram

Source: ESP

Power MOSFET Active Bridge Rectifier

The losses in a bridge rectifier can easily become significant when low voltages are being rectified. The voltage drop across the bridge is a good 1.5 V, which is a hefty 25% with an input voltage of 6V. The loss can be reduced by around 50% by using Schottky diodes, but it would naturally be even nicer to reduce it to practically zero. That’s possible with a synchronous rectifier. What that means is using an active switching system instead of a ‘passive’ bridge rectifier.

The principle is simple: whenever the instantaneous value of the input AC voltage is greater than the rectified output voltage, a MOSFET is switched on to allow current to flow from the input to the output. As we want to have a full-wave rectifier, we need four FETs instead of four diodes, just as in a bridge rectifier. R1–R4 form a voltage divider for the rectified voltage, and R5–R8 do the same for the AC input voltage. As soon as the input voltage is a bit higher than the rectified voltage, IC1d switches on MOSFET T3.

Plant Watering Watcher

A flashing LED signals the necessity to water a plant, 3V powered circuit
This circuit is intended to signal when a plant needs water. A LED flashes at a low rate when the ground in the flower-pot is too dry, turning off when the moisture level is increasing. Adjusting R2 will allow the user to adapt the sensitivity of the circuit for different grounds, pots and probe types.

Improvements:
This little gadget encountered a long lasting success amongst electronics enthusiasts since its first appearance on this website in 1999. Nevertheless, in the correspondence exchanged during all these years with many amateurs, some suggestions and also criticism prompted me to revise thoroughly the circuit, making some improvements requiring the addition of four resistors, two capacitors and one transistor.

Inverter Circuit For Soldering Iron

Here is a simple but inexpensive inverter for a small soldering iron (25W, 35W, etc) In the absence of mains supply. It uses eight transistors and a few resistors and capacitors. Transistors Q1 and Q2 (each BC547) form an astable multivibrator that produces 50Hz signal. The complementary outputs from the collectors of transistors Q1 and Q2 are fed to pnp Darlington driver stages formed by transistor pairs Q3-Q5 and Q4-Q6 (utilising BC558 and BD140).

The outputs from the drivers are fed to transistors Q7 and Q8 (each 2N3055) connected for push-pull operation. Use suitable heat-sinks for transistors Q5 through Q8. A 230V AC primary to 12V-0-12V, 4.5A secondary transformer (T1) is used. The centre-tapped terminal of the secondary of the transformer is connected to the battery (12V, 7Ah), while the other two terminals of the secondary are connected to the collectors of power transistors T7 and T8, respectively.

When you power the circuit using switch S1, transformer X1 produces 230V AC at its primary terminal. This voltage can be used to heat your soldering iron. Assemble the circuit on a generalpurpose PCB and house in a suitable cabinet. Connect the battery and transformer with suitable current-carrying wires. On the front panel of the box, fit power switch S1 and a 3-pin socket for connecting the soldering iron. Note that the ratings of the battery, transistors T7 and T8, and transformer may vary as these all depend on the load (soldering iron).

Circuit diagram:

Inverter Circuit Diagram For Soldering Iron

Bicycle Back Safety Light

Flashing 13 LED unit, 3V supply, Also suitable for jogger/walkers
This circuit has been designed to provide a clearly visible light, formed by 13 high efficiency flashing LEDs arranged in a pseudo-rotating order. Due to low voltage, low drain battery operation and small size, the device is suitable for mounting on bicycles as a back light, or to put on by jogger/walkers. IC1 is a CMos version of the 555 IC wired as an astable multivibrator generating a 50% duty-cycle square wave at about 4Hz frequency.

At 3V supply, 555 output (pin 3) sinking current operation is far better than sourcing, then LEDs D1-D6 are connected to the positive supply rail. In order to obtain an alternate flashing operation, a second 555 IC is provided, acting as a trigger plus inverter and driving LEDs D7-D12. D13 is permanently on. The LEDs are arranged in a two series display as shown below, with a center LED permanently on. This arrangement and the alternate flashing of the two series of LEDs provide a pseudo-rotating appearance.

Circuit diagram:

Bicycle Back Safety Light Schematic Circuit Diagram

Fading Leds

Two strips of LEDs fading in a complementary manner, 9V Battery-operated portable unit

This circuit operates two LED strips in pulsing mode, i.e. one LED strip goes from off state, lights up gradually, then dims gradually, etc. while the other LED strip does the contrary. Each strip can be made up from 2 to 5 LEDs at 9V supply. The two Op-Amps contained into IC1 form a triangular wave generator. The rising and falling voltage obtained at pin #7 of IC1 drives two complementary circuits formed by a 10mA constant current source (Q1, Q2 and Q5, Q6) and driver transistor (Q3 and Q6). R4, R5 & C1 are the timing components: the total period can be varied changing their values. R7 & R8 vary the LEDs brightness.

Circuit diagram:

Fading Leds Circuit Diagram

Dancing LEDs

LED sequencer: follows the rhythm of music or speech, 9V Battery-operated portable unit
The basic circuit illuminates up to ten LEDs in sequence, following the rhythm of music or speech picked-up by a small microphone. The expanded version can drive up to ten strips, formed by up to five LEDs each, at 9V supply. IC1A amplifies about 100 times the audio signal picked-up by the microphone and drives IC1B acting as peak-voltage detector. Its output peaks are synchronous with the peaks of the input signal and clock IC2, a ring decade counter capable of driving up to ten LEDs in sequence.

An additional circuit allows the driving of up to ten strips, made up by five LEDs each (max.), at 9V supply. It is formed by a 10mA constant current source (Q1 & Q2) common to all LED strips and by a switching transistor (Q3), driving a strip obtained from 2 to 5 series-connected LEDs. Therefore one transistor and its Base resistor are required to drive each of the strips used.

Circuit diagram:

Dancing LEDs Circuit Diagram

Automatic Low-Power Emergancy Light

Here is portable, simple and inexpensive unit white-LED-based emergency light that offers the following advantages. 1-It is highly bright due to the use of white LEDs. 2-The light turns on automatically when mains supply fails, and turns off when mains power resumes. 3-It has its own battery charger. When the battery is fully charged, charging stops automatically. The charger power supply section is built around 3-terminal adjustable regulator IC LM317 (IC1), while the LED driver section is built around transistor BD140 (Q2).

In the charger power supply section, an input AC main is stepped down by T1 to deliver 9V, 500mA to the bridge rectifier, which comprises diodes D1 through D4. Filter capacitor C1 eliminates ripples. Unregulated DC voltage is fed to input pin 3 of IC1 and provides charging current through D5 and limiting resistor R15. By adjusting preset P1, the output voltage can be adjusted to deliver the required charging current. When the battery gets charged to 6.8V, D6 conducts and charging current from IC1 finds a path throughQT1 to ground and it stops charging of the battery. When mains power is available, the base of Q2 remains high and Q2 does not conduct. Thus LEDs are off.