I found this circuit in the projects forum. It is a water level alarm for a pet water dish. The two transistors (Q1 and Q2) used to trigger the alarm are configured in an interesting way. They must keep the timer's trigger voltage high until the water level drops and breaks continuity, but I don't know how.
Could anyone offer a brief explanation? Thanks, MT.

 

They form a Sziklai pair - like a darlington arrangement but a PNP and an NPN transistor. It effectively gives you a transistor with a gain equal to the product of their individual gains.
https://en.wikipedia.org/wiki/Sziklai_pair

 

Q1,2 are turned on by the water in the dish, as long as there's water the 555 chip will keep pin3 high and no sound, when the water is empty, the transistors turn off and the 555 is allowed to oscillate thus making a sound.

 

Could anyone offer a brief explanation?

With water level high, continuity exists between the two probes so the transistors are conducting, shorting out the capacitor - alarm is not sounding.

As water levels drop, the continuity breaks and the two trsnsitors are no longer conducting and the 555 starts to oscillate - the alarm is sounded.

 

Thanks for the replies and info. Sziklai pair, very interesting. I'm looking at the wiki article now. The more I look at this circuit, the less sense it makes to me. With the high values of the RC circuit, the 7555 couldn't oscillate at an audible frequency (could it?). And, as long as the darlington pair provides a path to ground to keep the capacitor discharged, pin 3 will be on (right?). Wouldn't the alarm sound continuously? I will have to look at it again tomorrow with fresh eyes.

 

With the high values of the RC circuit, the 7555 couldn't oscillate at an audible frequency (could it?).

No it couldn't but the 'sonalert' is intended to be the kind which bleeps just with DC applied to it and the slow '555 just pulses the sound.

 

And, as long as the darlington pair provides a path to ground to keep the capacitor discharged, pin 3 will be on (right?) Wouldn't the alarm sound continuously?

No.
Pin 3 will be high which means there is no voltage across the sonalert since it is connected to V+.

 

The circuit designer is using a 2N2222A (Q1), and a PN2907A (Q2). It seems like, as long as the water level maintains continuity, there is a positive voltage drop across R4 which turns on the Sziklai pair providing a path to ground which will keep the 7555 output high. The configuration of the Sziklai pair is like an NPN overall. Oh! I see now. When the 7555 output goes low (current sink) the alarm sounds. I wasn't getting it. Thanks for the input.

 

a small signal mosfet, like 2n7000, would actually work better than a pair of bjt.

 

An alternative circuit - leave the timing components alone, connect the Q2 collector to the Reset pin 4 (after disconnecting pin 4 from the battery), and connect SPK1 to ground. As long as Q2 holds the 555 in reset, the output is low. With the sonalert's other end at GND, no sound.

ak

 

Excellent ideas. Originally, I was curious about the complementary transistor pair. After looking at the circuit, and getting great answers to my questions, I can see how a 555 chip can be used as an alarm trigger (with intermittent beeping set with RC). Either holding it in one of it's bi-stable modes, or keeping it in reset until some condition changes. Many possibilities for another widget to make for the grand kids.

 

I found this circuit in the projects forum. It is a water level alarm for a pet water dish. The two transistors (Q1 and Q2) used to trigger the alarm are configured in an interesting way. They must keep the timer's trigger voltage high until the water level drops and breaks continuity, but I don't know how.
Could anyone offer a brief explanation? Thanks, MT.


View attachment 116506

??? When the water dish runs dry Q1 andQ2 turn off, allowing the circuit to oscillate. While trigger is low the output is high, sonalert off. When trigger is released the circuit oscillates andwe get beeps out of the sonalert.