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Waterproof low voltage circuit needed

TFW

Mar 2, 2021
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I'm looking to switch on an indicator light, until reset by a manual switch when a voltage drop occurs. the circuit is typically 3.6-4.5V (DC) when this voltage drops to 0.7-1.4V (DC) I want it to trigger a LED indicator so we know this trigger has occurred. Even after all power has been removed from the circuit.

Basically it is a lean angle sensor on a piece of equipment that will be turned off and lose all power. I need a way to track if the machine has been tilted past this sensor.

What is the solution to this? thanks in advance
 

ChosunOne

Jun 20, 2010
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Okay, I'm stymied as to how an indicator light can remain lighted when all power to the circuit has been removed. :confused: I'm dying to see a solution to this one.

Now it the indicator could be some kind of mechanical flag, I might take a shot at it; but OP has stipulated an LED remain lighted after all power is removed.
 

TFW

Mar 2, 2021
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I should have been more clear. The machine has a 12v dc battery system. I’d run the LED off the battery. The tilt sensor drops the 1-5v range to 0 when the key is shut off. I’d want the indicator light to remain on even throughout key cycles. Then a switch to reset it to off - ready to be tripped again.
I’m open to a mechanical counter type style as well though.
 

AnalogKid

Jun 10, 2015
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The circuit is called a flipflop, which is a 1-bit latch. There are zillions of example schematics on the innergoogle. For a 12 V system, this can be done with one CMOS logic chip (actually, any one of several different types), or two transistors plus some resistors. Personally, I'd go with the classic two-transistor version, but I'm a classic (old) and have lotsa parts.

Tell us more about the sensor. Mechanical? electrical? electronic? Power supply voltage? Ranges of output voltages for both the OK and Tripped states? Any information about how much signal current it can source? Link to web page?

1. Were are you located?

2. What is your skill set for building a small circuit?

3. There are shock and tilt sensors that are purely mechanical. You attach them to sensitive equipment or material to verify that it was not mishandled during shipment. I've never used them, but remember seeing ads for them in Machine Design and other mechanical-oriented magazines.

ak
 
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TFW

Mar 2, 2021
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I’m located in WV.
I’m a novice compared to most of you I’m sure but I am comfortable in wiring 12v accessories etc.

I’ve used some mechanical solutions however they gave too many false positives. There is a lot of jarring and vibration, the machine already has the tilt sensor so I was looking to incorporate that sensor because it works very well to rule out false positives. It just doesn’t have a memory of how many times it was tripped.

the sensor now I’m not sure if it’s internals I just know it gives off a +/- 4V signal when it’s operated correctly and when it’s tilted past 65 degrees it gives a +/- 1V signal

I was thinking of incorporating some sort of latching relay but I can’t find any that trigger at 0.7-1.6V and don’t trigger at 3.6-4.5V
 
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AnalogKid

Jun 10, 2015
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Link to the sensor?

Your voltage level descriptions are not clear. To us, +/- 4 V is the way to describe a sine wave or some other periodic signal that goes both above and below GND.

I'm guessing you mean that the OK signal is approx 4 V or more above GND, and the Tilt signal is 1 V or less above GND. Yes / No ?

If yes, then this can be done with 2 transistors, or one transistor and an SCR, or a CMOS toggle flipflop, or or or.
It just doesn’t have a memory of how many times it was tripped.
This hasn't come up before. Another option is a counter that tells you how many trips since the last reset. This could be an ebay module - no circuits for you, just switches and wires.

Hmmm .. ?

ak
 

ratstar

Aug 20, 2018
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Yeh you need a latch to do it properly, you could make a cruddier one just by using a mosfet (cause they sorta act like a latch in themselves.), and u just check if its been charged or not, by getting a blip into an led, but u wont know until you check, u might need to amp it into a capacitor tho, cause the mosfet capacitor might be too small in capacitance.
 

TFW

Mar 2, 2021
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Link to the sensor?

Your voltage level descriptions are not clear. To us, +/- 4 V is the way to describe a sine wave or some other periodic signal that goes both above and below GND.

I'm guessing you mean that the OK signal is approx 4 V or more above GND, and the Tilt signal is 1 V or less above GND. Yes / No ?

If yes, then this can be done with 2 transistors, or one transistor and an SCR, or a CMOS toggle flipflop, or or or.

This hasn't come up before. Another option is a counter that tells you how many trips since the last reset. This could be an ebay module - no circuits for you, just switches and wires.

Hmmm .. ?

ak

this is what I meant. I need to trigger it at the lower voltage, but not at zero volts (when turned off) and not at its normal operating state of 3.6-4.5V
155577354_3812698125474003_142863756197004558_n.png
 

AnalogKid

Jun 10, 2015
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The circuit is called a window comparator. Typically this is done with two comparators, such as the two halves of an LM393. One senses that the input is below the upper limit, and the other senses that it is above the lower limit. These drive a small transistor that drives the LED.

How well-regulated is the 5 V power to the sensor? If the stability/accuracy of the 12 V or 5 V power is not good, then you need an a voltage reference of some type, like a zener diode or reference IC. All small, low-cost components.

Is there ever a time when the sensor is unpowered but the comparator circuit is? If no, then making the window circuit latch is fairly simple.

The parts will run on anything from3 V to 36 V. What is your preferred power supply voltage for the comparator circuit?

ak
 
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AnalogKid

Jun 10, 2015
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First pass at a latching window comparator with reset. U1A and B form the window comparator. The output is high only if the input is between the two trip points. Note that the high trip point voltage is used as the reference for the two following stages. Those stages need anything between 1 V and 3 V, and the 1.4 V is conveniently available with no extra parts.

U1C is an inverter, to drive the latch with the correct logic polarity. U1D is both latch and LED driver. Pin 11 goes low on error, which drives the output low, which holds the input low through D2. The LM339 outputs are open-collector, so when pin 14 tries to go high after a tilt has cleared, pin 11 stays low through D2.

When the circuit is latched, pin 11 is approx 1 V (pin 13 low voltage plus the D2 forward voltage drop) and pin 10 is 1.41 V through 100K resistor R7. Closing the switch pulls pin 10 down below pin 11, which drives the output high. Now, pin 13 is essentially an open circuit, pin 11 is a t 5 V, the LED is off, and the latch is reset. Releasing the switch raises the voltage at pin 10 to 1.4 V, still way below 5 V.

This circuit assumes that the 5 V rail is stable enough to be uses as the source for the two comparison threshold voltages.

ak
Sensor-Window-1-c.gif
 
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