555 / 556 H bridge

Posted on Feb 6, 2014

You might find yourself needing a low power H bridge for driving a motor like I once did. The 555 IC can drive a load up to 200mA, source or sink, which might make is usable as a driver, if one can control the output as desired. Although it is an unusual use for a 555/556 IC, it is an easy way to make a bridge with these easy to find and cheap ICs

555 / 556 H bridge
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. They are a good solution for a low power H bridge that operates at a higher voltage than the controlling circuit, up to 15V, requiring only an extra zenner diode. Two comparators control an SR flip flop which drives the output buffer. A high logic level on the Reset input makes the Output go low and a high logic level on the Set pin makes the output go high. In order to set the output, a voltage lower than 1/3*VCC must be applied to the Trigger pin and in order to reset it a voltage higher than 2/3*VCC must be applied on the Threshold pin. The 1/3 and 2/3 voltage references are given by the three 5K resistors. Connecting the two inputs (threshold and trigger) together means that the output may be set by a voltage under 1/3*Vcc and reset by a voltage over 2/3*VCC. Two of those circuits together make a H bridge: The 555 offers another advantage: the voltage reference 2/3VCC is connected to the Control voltage pin (see internal block schematic). That means that the two thresholds may be altered by controlling that voltage. If the motor requires a high voltage, such as 12V but needs to be controlled from a lower voltage microcontroller (3. 3 / 5V) the thresholds can be altered by forcing the control voltage pin to another level. Because a voltage higher than the control voltage pin level is required to reset the output, the control voltage pin should be pulled to a level that is some distance below the microcontroller supply....

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