Level switch for pump
In A Radio Practical Electronics Electus on p. 21 the connection of the level switch for the pump by Štěpán Burd was published. I decided to, build a similar circuit, but with a smaller number of components. The originals were used in total 3 transistors, 1x 7400, 1x78L05. I also used two level sensing positions. They were in my involvement 2 transistors, old germanium SFT125 and thyristor KT501. (I'm sorry not to use Ge transistors and throw them away).
Their h21e is around 300. However, any PNP transistors can be used, in the case of NPN, it is necessary to change the polarity of the power supply. PNP tr. I used therefore, so that there is a negative pole in the water – below. Resistors in bases have a value at T1 1k5, and for T2 which switches the control electrode of the thyristor, a larger value can be used, it is best to use a trimmer around 47k. For silicon transistors, the value of resistor T1 will be higher, satisfies about 2k2.
The supply voltage can be increased, but resistors and trans must be adjusted. they must be rated for this voltage.
It is also possible to insert an RC element in front of the relay, which limits the current. It is advisable to use a trimmer first and then replace it with a fixed resistor. The capacitor should have a capacity from 470n to 50uF depending on the type of relay. It will also be necessary to use a protection diode connected in parallel to the relay, to make an overvoltage pulse, which occurs when disconnecting power to the relay did not destroy you.
The power supply can also be temporarily connected to a phase conductor via a capacitor, obmädzovací rezistor (220), diode, filter condenser and zener. Capacitor capacity (connected to the phase) should start from 1uF / 250V (at 5uF was nap. už 250V!). All components with which I tested the circuit were at 250V, old drawer supplies will also suffice, for me it was mostly Russian (because the Russians have no pair!). Attention, it works with life-threatening voltage 230V and therefore caution is never enough. It is advisable to use an isolating transformer when setting.
It may be necessary in some cases to use tripping resistors in transistor bases (connected between base and + Ucc). A value from 10k to 100k should suffice. In addition, a filter capacitor can be mounted on the rectifier diode with a value of 10nF due to interference – commutation on this diode.
I tested this connection only on a universal PCB, but in practice the connection with the pump was not tested (we don't have water in the basement yet…)
Rado says goodbye
* We disagreed completely with the Council, whether such a source complies with STN or not in this application. Does anyone know the answer to this question responsibly? TNX, OM3-0122
