Submitted by: Arthur Pistorius (A.Pistorius@science.ru.nl).
For the remote control of a garden watering system, the use of a commercially available bi-stable water valve, operating at low voltage, was explored. Previous user experience with this type of device was disappointing. The driver circuitry is embedded in resin and it appeared difficult to bypass this controller. The valve is opened by a 50 ms pulse and is closed by a 10 ms pulse with reversed polarity. These pulses can be generated by two monostable multivibrators which are respectively triggered by the raising and falling edge of a common input signal. Alternatively, a small PIC microcontroller can be employed for timing of both pulses. These pulses are used to drive an integrated H-bridge power stage. Hereby, straightforward on and off control is obtained with a single I/O line, making it easy to combine it with a favorite microcontroller or webserver project as a replacement for the proprietary timer.
Photo 3: Working prototype. Topmost is the L293D H-bridge driver with 9V supply connection and coax output cable to the right. An HEF4538 dual retriggerable MMV is used to generate the opening and closing pulses. Level translation from TTL to 9 V is omitted in this setup.
Figure 3: Two monostable multivibrators, triggered on the raising and falling edge of a common input are used to generate pulses of different duration. In the non-retriggerable mode, the outputs are fed back to the opposite trigger inputs. The input signal is shortened by the two RC sections of 10k and 10 nF. Reversal of polarity is obtained by an H-bridge amplifier with integrated bypass diodes (one half of an L293D). Level translation from TTL to 9V is omitted in this design.
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