Simple switching circuit

On a sunny day (Sat, 5 Jan 2008 11:33:39 -0800 (PST)) it happened snipped-for-privacy@gmail.com wrote in :

Problem with circuits like this is, that if you make it more sensitive it will also start reacting to other light (environment). Perhaps you could add some posivive feedback, to make it flip over.

------------------------------------------------------ | | | | R1 3k3 R3 10k e | | |-----R4----- b T2 | | 10k c c PNP | |----R2------- b T1 | | | | e NPN | 100 c photo | | |---R7----b T3 transistor ----- | -----R5 ------| e | | see text | | | | | | | | R6 load | | | 2k2 | /// /// /// ///

When light falls on the photo transistor, then T1 will conduct, and so will T2. T2 will make T1 conduct a bit more via R5, R5 should be a high value, maybe 10 to 100x R2.. The idea is: Schmitt trigger

Or the transistor, more likely.

Hi Mick,

Your thinking is correct - you need a comparator with hysteresis.

You should also have a reverse connected power diode accross the pump terminals. Otherwise the transistor is likely to be destroyed by the turn-off spike from the motor inductance.

Someone here may sketch up a circuit for you, but you could also consider a relay as a quick solution. (This will need the diode accross its coil, too).

That is not a good circuit, the transistor does not saturate and will end up dissipating quite a bit of power. That 3.3K base transistor is not nearly enough drive for a 2.5A load, the output will current limit and the motor will not spin up. Assuming a motor resistance of about 1R, then a typical HFE of 100 makes the transistor look like 33 ohms in series with 12V-VBE for a stalled motor voltage of 11.2/34=0.3V and transistor power dissipation of 3.6W. That is not going to work very well. Your test load on the schematic is not representative of anything. You need to figure out a way to inject about 100mA into that base terminal before you do anything else. This will be left as an exercise.

On a sunny day (Sat, 05 Jan 2008 19:58:20 GMT) it happened Jan Panteltje wrote in :

PS You may want to add 1k from base T1 to ground. This will put the switch point at about 7 V ( (1 / 11) * 0.7)

Hi all,

Thanks for the suggestions. I've added a Schmitt Trigger, increased the current at the base of the output transistor, and added a diode to protect the output transistor. Any comments?

Thanks, Mick

Whoops, set the new design at

That's not nearly enough drive, and the switching levels are off. You need something BIG. Most switching applications using transistors place the load in the collector circuit, something like this, and an LM555 is the most convenient way to realize a Schmitt with maximum hysteresis: View in a fixed-width font such as Courier.

. . . . +12V . | . .-------+---------+---------------+------+---------. . | | | | | | . | | | | | | . | | | | [10,1/2W] | . | |/ | 555 47,000u|+ | |< . v ~~ | ---+--- === +-------| D45V10 . - |> | | | | |\\ . | | | out|----. | [15R,10W] | . | | | | | | | | . | +-+---|trig | [120,1W] | | +----> To motor . | | | | | | | |/ | . | | '---|thresh | +-----------| D44V10 - . | | | | | | |> ^ 3A . [Rc] [3.3k] | | [220] | | | diode . | | ------- | | | | . | | | | | | | . | | | | | | | . '-------+---------+--------+------+------+---------' . | . --- . ///

This one configures the 555 a little bit better: View in a fixed-width font such as Courier.

. . . . +12V . | . .-----+-----------+---------------+------+---------. . | | | | | | . | | | | | | . | | .--+ | [10,1/2W] | . | |/ 4| 8| 555 47,000u|+ | |< . v ~~ | ----+--- === +-------| D45V10 . - |> |rst V+ |3 | | |\\ . | | 6| out|----. | [15R,10W] | . | +----+-|thresh | | | | | . | | |2| | [120,1W] | | +----> To motor . | | '-|trig |7 | | |/ | . | | 5| dis|----+-----------| D44V10 - . | | .-|ctl | | | |> ^ 3A . | | 0.1| | gnd | | | | | diode . | | === -------- | | | | . [Rc] [3.3k] | |1 [220] | | | . | | | | | | | | . '-----+----+------+--------+------+------+---------' . | . --- . ///