Zener Diode Switches

How can I make a circuit that switches on at a certain voltage?

Making a solar robot: I need a circuit that:
- switches on at 9v
- STAYS on until it drops below 2v
- consumes as little power as possible, both while waiting to reach 9v and while going down to 2v

so a zener diode wouldn't work, because when we drop down below 9v it would stop conducting. Any ideas?

What you need is a comparator circuit. An output gets switched on or off at a certain voltage level. A comparator circuit generally uses an op amp and a reference voltage. You might actually use a zener diode to generate that reference voltage.

Do a search google search on voltage comparator and you should find some simple circuit diagrams to get you started. And btw, there are zener diodes that operste well below 9v.

Good luck!

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Dynasty 959.MOV

help zener diode and transistor as a switch?

zener diode

explain in good condition
VA=+18V
VB=+10V
VC=+10V

Find the trouble and discuss
1. VA=+18V
VB=+10V
VC=0V

2. VA=+18V
VB=0V
VC=0V

transistor as a switch

1.determine whether the transistor is saturated. prove.
2. if Vcc is reduced to +5V what happens to the transistor?
3. if Rb is reduced to 33K ohm what happens to the transistor?
..............

Zener diode:

1. VB and VC are connected together in the schematic, so because VB is greater than VC, and VC is downstream from VB, then the connection between them is broken.

2. There is a short circuit between VC and ground. Most likely, this would equate to a shorted zener diode, but it could be any short that would exist between VC and ground.

Transistor as switch:

1. The transistor is not saturated, as the transistor is not conducting sufficiently to sink the maximum collector current. The Beta is 100, which is the collector current divided by the base current. The circuit has the base current set too low (9.3V / 1,000,000 ohms) to allow the collector to conduct the full potential of the collector circuit (10V / 3300 ohms).

2. You did not identify which resistor was Rb. But if you change the 1M resistor to 33k, then the transistor would saturate in conduction. Now the base current times the beta is greater than the maximum collector current possible.