Pushbutton to use with Powerboost 500C

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harleyfrazee
 
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Pushbutton to use with Powerboost 500C

Post by harleyfrazee »

I use a 1/2" pushbutton connected to the EN/GND pins on the Powerboost 500C in my project. The problem is that all the SPST latched pushbuttons I can find are normally open so you push the button and the EN/GND pins are shorted and the power goes off. So, I have a power-off button (not a power-on button). I've searched all over and can't find an affordable, small normally closed pushbutton. I would like someone to recommend an appropriate pushbutton or the minimal circuit that can reverse the action of the EN/GND

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dastels
 
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Re: Pushbutton to use with Powerboost 500C

Post by dastels »

I just wired this up to test it. Works fine. Vs and EN are on the Powerboost.
pushbutton-enable.jpg
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harleyfrazee
 
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Re: Pushbutton to use with Powerboost 500C

Post by harleyfrazee »

You're amazing. Thanks for the circuit. I only had NPN transistors around. I've ordered the 2N3904 and should get it tomorrow and I'll let you know. Also, with the current project, the pushbutton switch is 30" away from the EN pin of the Powerboost 500C. I'm making a mechanical change to put the switch within 4" of the EN pin. I assume that's necessary.

Finally, This circuit is powered directly from the 3.7V (4.1V to 3.3V) 18650 Lipo battery. I was wondering how you calculated the 270 Ohm resistor.

Thanks again
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dastels
 
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Re: Pushbutton to use with Powerboost 500C

Post by dastels »

2N3904s are NPN. 2N2222s shoudl work as well.

As usual (other than making antennas) the sorter the wires, the better.

The resistor value is somewhat arbitrary. You just need something to limit the current somewhat. But you still want enough current to saturate the transistor.

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harleyfrazee
 
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Re: Pushbutton to use with Powerboost 500C

Post by harleyfrazee »

Sorry. I messed this up when I replied your circuit worked for me

The EN pin of the PowerBoost 500C is a bit backward from what I thought it should be. When the EN pin is open (not shortened to ground), the PowerBoost is turned on. To turn the PowerBoost off, you need to ground the EN pin.

What I wanted to happen with my product was that when you press the button in, the EN pin is open and when you push the button again and it is open (latching) the EN pin is connected to ground.

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dastels
 
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Re: Pushbutton to use with Powerboost 500C

Post by dastels »

I misunderstood a bit as well. Slight change to control of the transistor. With the switch open, the transistor is held "on" by the resistor, which grounds EN, turning off the powerboost. With the switch closed, the base of the transistor is grounded, turning it off. That lets EN float, turning the powerboost on.
powerboost-control.jpg
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dastels
 
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Re: Pushbutton to use with Powerboost 500C

Post by dastels »

Though the best/easiest solution is to use a normally-closed latching pushbutton (or a slide/toggle switch).

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inspectorgadget
 
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Re: Pushbutton to use with Powerboost 500C

Post by inspectorgadget »

There's some waste in using a bipolar transistor as a power switch inverter because in the off state it will be consuming 3V / 10K = 0.3mV through the base bias. This will self-discharge a 500mAh LiPo in 1600 hours, or about 2 months. It would be better to use an NFET with a 150K pull-up which extends your drain to about 2 years.

Or you could use the Adafruit tactile power switch module which directly switches your power line on/off without a latching button. It's an extremely low power consumption circuit with a bistable gate circuit.

Product Page: https://www.adafruit.com/product/1400
Circuit: https://cdn-shop.adafruit.com/datasheets/1400sch.png

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harleyfrazee
 
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Re: Pushbutton to use with Powerboost 500C

Post by harleyfrazee »

When I tried using the circuit with the 2N3904, as shown in the schematic below, the Lithium battery immediately heats up. Is there something within the Powerboost that is shorting between the battery+ battery-?
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