Latching switches?

Discussion in 'The Projects Forum' started by kristel, Oct 4, 2011.

1. kristel Thread Starter New Member

Oct 4, 2011
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Other than what I have learned from my basic circuits course, I do not know much about the topic in general.

I have a project that my boss had given me today JUST before I left work. And, he expects me to have a sketch/plan by Thursday to present at a meeting. It isn't a formal presentation or anything, but I am stuck. So please help if you can.

The dilemma is that we need to replace a photodiode, which we use to turn on the system in our little project. The photodiode is programmed to sense infrared light to turn on, however, as infrared light is also present in any visible light, if the system isn't in the complete dark, it depletes our battery. (We have to have it running on standby/off for approx. 2 years, so you can see the issue.)

A dome-like silicone cover encases our circuit board and battery. What we may want to do is replace the photodiode with a switch. Perhaps a simple membrane key that, when pushed, can have a wire touch the battery to activate it? (Not sure, as when he was explaining, his hand was covering his very vague diagram.)

I considered maybe using a latching switch? But, as I know almost nothing about circuits, at least not in an innovative sense, I was hoping I could get your input.

Thank you!

If the sketch is needed I will be happy to provide it tomorrow... and by sketch I mean the one my boss did... I'd reproduce one but I'd have to understand it first.

2. SgtWookie Expert

Jul 17, 2007
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You didn't mention what voltage you had available, so I just assumed it was a 9v battery. You also didn't mention the circuit current requirement, so I assumed that it is quite low; a few 10s of mA

Here's one way to do it:

S1 can be a tactile switch. They are small, inexpensive and reliable. R1 ensures that the switch doesn't have high momentary load currents, which could shorten its' useful life. D1 may or may not be necessary; as I mentioned above, I have no idea what the rest of your circuit is.

The circuit itself will need to be responsible for keeping the PwrCtrl output high for the duration that the circuit is supposed to be operating. Current draw will be minimal.

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3. kristel Thread Starter New Member

Oct 4, 2011
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SgtWookie,

Thanks for the reply. I truly appreciate the help. Sorry I didn't include that info. I didn't have it readily in front of me at the time I posted. The battery is 3.6V Lithium. And not entirely sure of the current... searching through docs atm and can't find any info on that. However, it is a device that will output electrical impulses in a range of 0 - 22 volts... Not sure if that helps or not.

4. SgtWookie Expert

Jul 17, 2007
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Is the device supposed to power down automatically under its' own control, and/or by pressing the same switch, or by pressing a different switch?

[eta]
The tactile switch I mentioned could also be a membrane switch as you mentioned in your first post. Whatever switch you decide to use, only very little current through the switch itself will be needed. The circuit current is controlled by the MOSFET.

With the lower available voltage, you will need a low-threshold MOSFET. The BSH103 should work for that:
http://www.nxp.com/documents/data_sheet/BSH103.pdf

Last edited: Oct 5, 2011
5. ErnieM AAC Fanatic!

Apr 24, 2011
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After not looking very hard I came across a Vishay Si3460DV, which is a logic level MOSFET that has very decent turn on characteristics (<.04 ohms at 12A) with only 1.8V on it's gate. So you should be able to use SgtWookie's idea with such a device.

When off, these things leak about 1 uA so see how that plays into your battery lifetime. But even given a 1A hour battery, 1 uA gives 114 years of life.

You just need an extra signal from your controller to drive the device.

6. SgtWookie Expert

Jul 17, 2007
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Kristel wrote via Email:
It's too bad that you sent your inquiry via E-mail, as it delayed getting a response by nearly a full day.

The MOSFET switch does not generate current within itself, and the device I suggested presents a leakage rate that is less than 1/10 of 1 microampere.

The MOSFET scheme I suggested will require virtually no power while on standby. It does require someone pressing a button on the unit itself in order to turn the MOSFET gate on, but that is what you communicated in your initial post; a switch. Perhaps in your thought process, you were thinking a switch that could be remotely activated yet use no power - unfortunately you did not say that in your initial problem statement.

I don't know offhand of a circuit that would react to an RF remote control without it being powered - except for a crystal radio set. However, the world today is so electrically noisy that it would be difficult to distinguish between an RF transmitters' legitimate commands, and random noise, without having a powered receiver - not to mention the difficulties with RF emissions regulations worldwide (they vary depending upon the country).

7. SgtWookie Expert

Jul 17, 2007
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Here is a document that you may find helpful; it's regarding passive RFID (wireless sensor) tags that can be read using a SAW (surface accoustic wave) device.

The idea here is that the passive remote device draws all of its' power from the emitted radiation; in your case it would be a remote control device sending pulses to an RFID tag that would use the power to "wake up" and perform an active task.

It's somewhat akin to an old-fashioned crystal radio set; those were powered by the radio station itself; there was just enough power available to cause sound in the headphones.

http://prod.sandia.gov/techlib/access-control.cgi/2006/061288.pdf