I know relatively little about electronics, and attempting to set up a circuit thus: A 9vdc battery powers a small air pump. When the voltage drops below 7.9 volts, it should shut off (ie, once air pressure is achieved.) When voltage passes 7.9 (or a slightly higher #), it should click on again, automatically.
A similar topic (1) was about the same thing, and using a comparator, or "window comparator" was suggested. I've found a couple good info sites through google, but am still unsure. Is what I want even possible? If not, how about a current comparitor instead?
If you set both the turn on and turn off voltages the same (in this case 7.9 volts), the device will cycle on/off forever, or until something breaks.
Hysterisis, as was mentioned in another reply, can help to avoid that, but there may be a different design problem. Controlling air pressure by reacting to the battery voltage level as you described is not a good idea, if you are looking for some specific pressure. What are the actual design goals? Control air pressure? Protect the battery? What causes the battery voltage to rise?
"pEhrlich" wrote in news: snipped-for-privacy@l53g2000cwa.googlegroups.com:
If using an op-amp (maybe same for dedicated comparator IC), put a reference voltage on the inverting input, the voltage you're watching on the non-inverting input (reverse these if you want to switch on when that would be switching off), and put a high resistance (try 1 meg or more) between the output and the non-inverting input. That resistance adds hysteresis, it prevents erratic switching when the two voltages are very close together.
I posted this message before, but it seems to have failed. Here goes again...
I've been able to start soldering, and to study more, and I've come up with a couple of problems.
When the air pressure rises, the input voltage drops as it becomes harder to pump. However, as the reference voltage is set by the same battery, won't they drop together? This could be fixed by using two batteries.
There is a much bigger and more fundamental problem. When the motor is turned off, the battery voltage will jump back up to 9v, causing it to switch back on. (this would loop until the air pressure lowers again). This can't be simply solved by having two supplies. Unless there is a clever solution to this floating around out there, it looks like a pneumatic pressure microswitch is the way to go.
I posted this message before, but it seems to have failed. Here goes again...
I've been able to start soldering, and to study more, and I've come up with a couple of problems.
When the air pressure rises, the input voltage drops as it becomes harder to pump. However, as the reference voltage is set by the same battery, won't they drop together? This could be fixed by using two batteries.
There is a much bigger and more fundamental problem. When the motor is turned off, the battery voltage will jump back up to 9v, causing it to switch back on. (this would loop until the air pressure lowers again). This can't be simply solved by having two supplies. Unless there is a clever solution to this floating around out there, it looks like a pneumatic pressure microswitch is the way to go.
"pEhrlich" wrote in news: snipped-for-privacy@k78g2000cwa.googlegroups.com:
Don't rely on the battery voltage to tell you that the pressure has changed. Your observation is good, but look further. It depends on the battery type, state of charge, and probably other variables, too many to get an accurate measure.
You're right that the reference could change, and that you need a fixed one. Look up zener diodes if you're not familiar. That would be better than nothing, but ideally you need something that directly converts a pressure reading to an electrical signal. There might be cheap ways to do this, and if you only need an on/off command based on a threshold, you might do ok with a microswitch instead of a comparator circuit, as you suggest.
If you have access to the pump drive shaft (assuming it's a motor), you could paint a black blob on it, and use a detector based on an LED and a photodiode to detect speed. Those parts come in various forms, and are often found in old printers, disk drives, scanners, all sorts of stuff. Converting the pulse train to an analog voltage you can measure is easy to do. If you ever want proportional control, this might be the way to go.
Ok, so the goal is to control pressure, not just protect the motor. Use 2 pressure switches - one for low to turn the motor on, and one for high, to turn it off. Below is a complete diagram, but since the pressures are unspecified, you will need to determine if the switches in the parts list at the bottom will work for you.
How it works: When turned on, both HP and LP switches are open. The PNP transistor is biased on by the 1K resistor to ground. The NPN transistor is turned on by the 1K resistor to +9, which energizes the relay, closing the RY1-1 contacts. When pressure builds up higher than the lower limit, the LP switch closes, removing the + bias from the NPN and turning it off. However, the RY1-1 normally open contact maintains a path to ground for the relay coil, so the relay stays energized. Pressure continues to build, until the HP switch transfers, removing the negative bias from the PNP, which turns off. That causes the relay to de-energize, and the motor stops. Pressure decreases, and the HP switch opens, removing the + from the PNP base, so it can turn on. However, there is no path to ground, as the relay is de-energized and the NPN is held off by the closed LP switch, until the pressure drops below the lower limit. That turns on the NPN, and the cycle repeats. The 47 uF cap and D2 keep the voltage to the transistors stable when the motor switches on and off.
Parts from Allelectronics:
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Cat # PSW-12 (pressure switch, $1.00 each) and Cat # RLY-642 6 volt DPDT relay. The 33 ohm resistor will drop the 9 volts down so the relay will work fine. They also sell the transistors (any PNP and NPN will work), cap, resistors and 1N4002 diodes.
You may need to make TEE fittings to connect the tubing to the switches, with one leg of the TEE going to a piece of tubing where you can bleed off some of the pressure to get the switch to operate at the right level.
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