| 摘要 |
<p>An electrical bridge circuit having a conductivity cell in one leg thereof, normally biased away from the null point so that a signal is produced by the bridge when the conductivity cell is operating at a desired set point. A semiconductor switching circuit is attached to a solenoid valve to vary material added to a solution, such as soap mixed with water. A combination of switching circuit bias, the signal from the bridge and a signal platform produced by a zener diode during portions of the positive cycle of an alternating power source, normally maintain the switch in a non-conducting state but sufficient variation of the conductivity cell in a direction caused by low conductivity, or too little material in the solution, produces a sufficient signal to switch the semiconductor switch into a conducting state to operate the solenoid valve.</p> |
