• Alternating CW & CCW Motor Driver
  A while back a company needed a motor controller for a custom door latching system.  The system used a small brush motor with a speed reduction gear to both latch and unlatch the door.  When pulsed for about 250ms in the clockwise direction, the motor would unlatch the door.  When spun in the opposite direction for 250ms, it would latch the door. The circuit below performs this function.  A single low power pushbutton is used to alternate between the two directions.  The circuit is powered by 4.5v using a battery made from three alkaline AA cells.  The motor draws about 250ma. The battery should provide many years of service.

• Constant Current LED Driver 5
  LEDs are current driven devices.  The light they produce is proportional to the current flowing though them.  One way to maintain a fairly steady light output is to keep the current through the LED fixed, even as the supply voltage may change.  A classic way to do this is shown below.  This works fairly well but it has two weaknesses.

• Low Voltage Latching Relay Driver
  Using some small super capacitors, this circuit can latch and unlatch a mechanical relay with 10 amp contacts, from a small 3 volt power source.  By using a latching relay, power can be controlled to a load with a tiny battery.  (added 7/06)

• Medium Power 40kHz Ultrasound Transducer Driver
  This crystal controlled circuit drives a 40KHz piezoelectric transducer with a 30v peak to peak signal.  (added 7/06)

• N-CH & P-CH Transistor form Push-Pull Driver
  This circuit can produce high speed output signals with fast rise and full times. The unique change pump

• ON/OFF Switch with Latching Relay
  The circuit below switches AC or DC power to a load using an ultra low power 3v latching relay with two coils.  The relay can handle about 10 amps of current, up to 250vac.  Each time the small pushbutton is pressed, the circuit either latches the relay contacts closed or unlatch them open.  The 3v powered circuit uses a single 74HC14. This device contains six Schmitt trigger inverters in the one 14 pin package....

• Pot Controlled Variable LED Intensity Circuit
  The circuit below uses power from four rechargeable AA NiMH cells to drive 3 white LEDs.  A potentiometer varies the duty cycle of a pulse width modulator circuit to vary the intensity of the LEDs from 0% to 100%.  The beauty of the circuit is that when the pot is all the way toward 0%, very little power is drawn from the battery.  Likewise, when the pot slider is moved toward the 100% end, full power is fed to the LEDs.  In effect, the pot becomes the on/off switch....

• Ramping Lamp Driver
  This circuit drives any 12v lamp, incandescent or LED type, in a ramping fashion.  Each press of the button toggles the lamp from a fully off mode to a fully on mode.  But, instead of an instantaneous lamp intensity change, the power is slowly ramped up and down.  With the components shown, the ramping action takes two seconds to complete.  By changing the capacitor C3, you can increase or decrease the ramping time.  The circuit will also operate with supply voltages ranging from 15v to 3v.

• Ramping LED Driver
  The circuit below was designed to drive a LED with an intensity ramping mode.  Two 555 timers generate two different triangle waveforms.  The upper device generates a 10KHz signal while the lower unit produces a 1Hz signal.  The two signals are fed to a voltage comparator.  The result is a pulse width modulation (PWM) signal, which with the aid of the FET, drives the LED in such a way that its average light output slowly ramps from about zero light to maximum and then slowly dims back down.  The circuit should operate over a supply voltage ranging from 3v to 12v. You can easily vary the ramping time by changing the value of the 1M resistor.  For an interesting effect, you can place a 1N4148 diode in parallel with the 1M resistor, with the cathode (banded end) side connected to pin 3.

• Reduced Power Relay Driver
  Relays can handle a lot of power.  However, for certain power sensitive designs you would like to reduce the power needed to hold a relay closed.  The circuit below performs such a task.  It uses a single CD4093 quad NAND gate.  When the “on” logic input signal is detected, the relay is first pulsed on for about 500ms. This is sufficient time to insure the relay is fully closed.  After that initial pulse the relay is then driven with a square wave signal, whose duty cycle can be adjusted.  The signal duty cycle can be adjusted from about 10% to 90%.  In most cases a 50% duty cycle will hold the relay closed. This reduces the average DC current required by the same factor, which means a 4:1 reduction in power. The circuit can operate over a wide 3v to 15v range.

• Square Wave Driver Has Flexible Outputs
  This circuit can produce an output signal ranging from DC to 100KHz. It can source a voltage ranging from 1v to 30v. It can sink a voltage ranging from zero volts to –30v. It can drive up to 200ma of current and can even be switched to a floating tristate output.

• White LED is Powered by One NiMH Cell
  There are a lot of white LED driver circuits floating around.   A popular circuit called the “Joule Thief” can drive one white LED from a single 1.2v or 1.5v battery cell.  Most of these circuits use one or two transistors to form a voltage boost circuit.  Yes, they those circuit do drive a LED but they are not very efficient and they don’t do a good job of controlling the current to the LED.  The circuit below uses a tiny LT1932 IC made by Linear Technology.  This IC can be configured for a wide variety of LED driver needs.  I have shown one simple application.  The circuit takes power from one 1.2v NiMH rechargeable battery or a 1.5v disposable battery and drives a single white LED with 15ma of current.  With an efficiency of about 70%, the circuit should run for about 40 hours, if a quality 2500ma-hour battery is used.

Driver Circuits

Master Category List - Dave's Circuits