Simple Voltage to Current Converter Drives White LEDs

You sometimes need to drive a white LED from one 1.5V battery. Unfortunately, the forward voltage of a white LED is 3 to 4V. So, you would need a dc/dc converter to drive the LED from one battery. Using the simple circuit in Figure 1, you can drive one white LED or two series-connected green LEDs, using only a few components. The circuit is a voltage-to-current converter, which converts the battery voltage to a current that passes through the LED.

You can adjust this current and, thus, the brightness of the LED, by varying resistor R3. If you turn on switch S1, resistor R2 feeds base current to transistor Q2. Q2 turns on, and its collector current, via R3, turns on Q1. Now, the current through inductor L1 increases. The slope of the increase is a function of the value of L1 and the battery voltage. The current through L1 increases until it reaches a maximum value, which depends on the gain of Q1. Because the value of R3 sets the base current drawn from Q1, Q1s collector current is also limited.

White LEDs Circuit Diagram

Once the current through L1 reaches its maximum value, the slope of the current through L1 changes. At that instant, the voltage on L1 switches to a negative polarity forced by the changed slope. This negative voltage traverses capacitor C1 and turns off Q2, which in turn turns off Q1. The negative voltage on L1 increases until it reaches the forward voltage of the LED. The peak current through inductor L1 now flows through the LED and decreases to zero. Now, Q2 switches on again, via the current through R2, and the cycle starts again.

By adjusting resistor R3, you can set the peak current through L1 and the peak current through the LED. The brightness of an LED is a linear function of the current through the LED. So, adjusting the value of R3 also adjusts the brightness of the LED.

It doesnt matter which LED you use; the forward voltage on the LED always increases until the peak current through L1 flows through the LED. Different forward voltages of the LEDs yield different on-times (duty cycles) but the same peak current through the LED. With the values shown in Figure 1, the circuit oscillates at a frequency of approximately 30 kHz and delivers a 20-mA peak current through the LED.

The duty cycle depends on the ratio of the battery voltage to the forward voltage of the LED. One advantage of this circuit is that it requires no series-limiting resistor for the LED. The peak current through the LED is a function of the value of R3 and the gain of Q1.[via]

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Simple Wireless TRIAC Dimmer

This project was used as a wireless light dimmer, but in principle can be used to dim resistive loads and wirelessly turn on/off loads. The current code includes a routine to dim a light bulb in a “heartbeat” pattern, with the heartbeat frequency remotely adjustable.

The top left of the schematic shows the wall outlet (US 120VAC) being stepped down with a small transformer, then full rectified and regulated. This powers the entire board from the wall. The top right shows a microcontroller, ATmega48, its programming header, and a UART connection to the microcontroller (for debugging). The bottom right shows the XBee and its basic voltage regulation (it’s 3.3V), as well as an LED that indicates when the XBee is connected.

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FM Transmitter Bug Circuit

Circuit Diagram 

Notes:

 This small transmitter uses a hartley type oscillator. Normally the capacitor in the tank circuit would connect at the base of the transistor, but at VHF the base emitter capacitance of the transistor acts as a short circuit, so in effect, it still is. The coil is four turns of 18swg wire wound around a quarter inch former. The aerial tap is about one and a half turns from the supply end. Audio sensitivity is very good when used with an ECM type microphone insert

  

Author: David, radio_david@yahoo.com
Source http://www.electronics-lab.com/

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Solid State Relay Required Only 50uA Drive Current

This circuit demands a control current that is 100 times smaller than that needed by a typical optically isolated solid state relays. It is ideal for battery-powered systems. Using a combination of a high current TRIAC and a very sensitive low current SCR, the circuit can control about 600 watts of power to load while providing full isolation and transient protection.

Solid State Relay Circuit diagram:

 Source: Discover Circuits

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Touch Sensor Switch Using Inverters

This touch sensor switch can is designed using inverters (N1, N2)and some common electronic components. In standby state at the entrances of N1 there is a signal produced by oscillator N3/N4. At the touch sensor hand capacity forms a bridge to the ground for the 1MHz signal so that the voltage signal at the entrance of N1 decreases more (at the exit of N2 is logical 1). After the release of contact, a signal charge C4 through D1 Mhz, so the output of N2 is 0 logic after short time.

Touch Sensor Switch Using Inverters Circuit Diagram

Installation can be powered with a DC voltage between 3 and 15 volts (maximum current of 2 mA is absorbed).

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Subwoofer Filter and Low Pass Filter with LM741

The acoustics of converting a filter, there are many aspects of the economic viability of the more famous are baxandal filter low and high frequency filters and crossover Acoustic space is transformed into sub-domains, so that the Thursday Speakers. Applications, we offer a filter, the limits of the region to transform acoustic (20-20000Hz) in the region of 20-100Hz.

Subwoofer Filter and Low Pass Filter Circuit Diagram

The signal for a first high pass filter C1, C2, P1, which is undesirable level DC input. A lowpass filter consisting of R3, R4, C3 prevents frequencies above 10 kHz, which do not benefit from this design, and it would be that the instability and noise. The summary amp invert signal.

The low Summary of the amplifier signals go to a second low-pass filter to prevent the frequency from the speakers. I decided, a second order, as this box with a closed place feature. If you have a circuit with a valve system, and then simply close the Wind (Roll a pair of socks and pick at the port / Wind), this will give you a sealed box instead.

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Remote Solar LED Light

I created this circuit in an attempt to make the simplest possible solar powered project. It would make for an excellent science fair project, and would also serve as a good introduction to solar powered circuitry. It may also have some practical uses, such as shedding some light into a dark part of your house. The idea is simple, the solar panel converts sunlight into a trickle of electricity. The electricity is used to run a white LED.

 Remote Solar LED Light  Circuit Diagram

Specifications:

• Operating Voltage: 3.7V DC
• Solar Current: 25ma max.
• LED Lamp Operating Current: 10-25ma.

Theory:
The remote solar powered LED light takes advantage of the current limited nature of solar photovoltaic cells. If light shines on the solar array, current will flow through the circuit. For a typical size of solar cell, there is a maximum current that can be produced. The maximum solar cell current is simply matched to a value of current that the LED can handle. If there is enough light to raise the solar panels voltage above around 3.7V, the white LED will light up. The LED regulates the maximum voltage across the circuit to around 3.7V.If the solar panel that you use produces more than 20ma, it may be necessary to insert a series resistor between the LED and the solar panel to prevent the LED from burning out.

A 50 ohm 1/4 watt resistor is probably about right for the job, the exact value may need to be optimized according to the solar panel that you use.This concept could easily be expanded to systems with larger arrays of solar cells and more LEDs. The capacitor is not required, but it will keep the LED from flickering if the panel is briefly blocked, such as when a bird flies by. With 7 solar cells, the LED will only light in fairly bright light, if you use up to 10 solar cells, the circuit will work nicely in overcast skies.For an interesting modification to this circuit, replace the 1000uF capacitor with a 1 Farad/5.5V "Memory Backup Capacitor". An Elna DB-545D105 device was tested on the circuit, after charging up in the sun for a few minutes, the capacitor was able to light the LED for several minutes.

Construction:
Most of the work goes into making the solar panel. Lay out the cells in any pattern. Cut the two pieces of plexiglass and one piece of perforated circuit board so that they are wider than the solar array. Stack the three board layers together and drill holes for the mounting screws. When the project is finished, the center circuit board will be spaced away from the front and back plastic panels with extra nuts acting as spacers on the mounting screws. The idea is to get an air gap above and below the circuit board so that there is room for the solar cells and wiring.

Mount the solar cells on the perf board and solder them into a series string. An easy way to do this is to connect short segments of bare wire-wrap wire to each cell, route the wires through the perf board and solder the ends on the bottom. Connect two wires to the ends of the series string of cells and secure the wires to the circuit board. For outdoor applications, seal the edge of the panel with silicone caulk or other water proof material. Also, seal the mounting screws where they pass through the plexiglass.Connect the LED and capacitor in parallel, wire them across the two power leads. Be sure to get the polarity correct, otherwise the LED wont light up. Solder the parts together. Be sure to heat-sink the LED leads while soldering, LEDs can be easily destroyed with too much heat.

Use:
Place the solar panel in the sun, the LED will light. The photo at the top of this page shows the circuit operating indoors on a cloudy day. If you put the LED on a long wire, it can be placed in a dark location, such as a corner of your basement. As long as there is a fair amount of light in the sky, the LED will light up. To get the best orientation for the panel, aim it directly at the sun at noon during March or September.

Parts:
7-10x photovoltaic cells, rated at 15-25ma each.
1x white LED, high efficiency types work best.
1x 1000uF 15V (or greater) electrolytic capacitor.
1 piece of perforated or printed circuit board.
2 pieces of clear plexiglass.
28 gauge bare wire-wrap wire.
24 gauge speaker wire.
miscellaneous screws, nuts, and washers.
silicone caulk.

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