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by Tage
Wed May 29, 2013 1:39 pm
Forum: Automation, sensing and robotics
Topic: Battery Power
Replies: 12
Views: 4721

Re: Battery Power

LiPO batteries with built-in electronics are relatively safe to use. But I would strongly advice against using single cells without the mandatory protections circuitry that is included in battery packs. Especially when using home made chargers. It only takes that you crank up the charge voltage a ti...
by Tage
Wed May 29, 2013 1:17 pm
Forum: Automation, sensing and robotics
Topic: Controlling 12VDC from GPIO
Replies: 25
Views: 14002

Re: Controlling 12VDC from GPIO

your schematic is correct except for two mistakes: you should remove the mechanical switch, and if you want an LED to light up when the solenoid is ON, you should but that LED in parallel with the solenoid coil, (first, add a resistor in series with the LED). Your circuit shows all three countermeas...
by Tage
Wed May 29, 2013 3:06 am
Forum: Interfacing (DSI, CSI, I2C, etc.)
Topic: Heartbeat from GPIO to safely drive relay
Replies: 6
Views: 2472

Re: Heartbeat from GPIO to safely drive relay

Here is a simulation schematic that shows the circuit and a couple of waveforms. The top waveform shows the voltage applied to the relay (RLOAD in the schematic). The bottom waveform shows the voltage across the 0.1uF capacitor. When the npn transistor stops switching the relay turns off. V1 represe...
by Tage
Wed May 29, 2013 2:44 am
Forum: Automation, sensing and robotics
Topic: Battery Power
Replies: 12
Views: 4721

Re: Battery Power

I noticed that it is possible to upload attachments to this forum -neat! I have to test that right now..
Here is a picture of a typical 18650 cell with solder tabs. This type of cell is used extensively even in large batteries for electrical cars.
Tage
by Tage
Wed May 29, 2013 1:40 am
Forum: Interfacing (DSI, CSI, I2C, etc.)
Topic: how to trigger gpio pin with 24v
Replies: 31
Views: 43590

Re: how to trigger gpio pin with 24v

May I suggest this solution: use optocoupler 4N28 or similar. it will cost you about $0.25 if you only buy a few. connect 10k resistor from GPIO to GND connect collector/emitter of optocoupler to 3.3V/GPIO connect 6.8k resistor in series with diode of optocoupler this should give you 3.3mA in the di...
by Tage
Tue May 28, 2013 11:17 pm
Forum: Interfacing (DSI, CSI, I2C, etc.)
Topic: Heartbeat from GPIO to safely drive relay
Replies: 6
Views: 2472

Re: Heartbeat from GPIO to safely drive relay

You could try a small npn transistor. Connect it across a 0.1 uF cap that has a 100 k resistor pullup to 12V. Each time the npn turns on it will discharge the cap. If it stops turning on the cap will charge to +12V. Connect Source of a p-ch MOSFET to 12V with the gate to the cap. Connect the Drain o...
by Tage
Tue May 28, 2013 1:58 pm
Forum: Automation, sensing and robotics
Topic: Controlling 12VDC from GPIO
Replies: 25
Views: 14002

Re: Controlling 12VDC from GPIO

If you have everything including the solenoids within a meter distance from the Pi, then the simplest way to protect the MOSFET against overvoltage is to add a diode (any diode, but 1N4148 is commonly used) directly across the solenoid. The cathode to the +12V end and the anode to the other end of t...
by Tage
Mon May 27, 2013 9:15 pm
Forum: Automation, sensing and robotics
Topic: Controlling 12VDC from GPIO
Replies: 25
Views: 14002

Re: Controlling 12VDC from GPIO

I certainly agree with the warning against using the Darlington array. With only 3.3V drive voltage from the GPIO there is barely enough drive current into the input to drive 300mA, and the voltage drop across the transistor is significant, in the order 1.5V...2V. So you could only drive a couple of...
by Tage
Mon May 27, 2013 2:39 pm
Forum: Automation, sensing and robotics
Topic: Controlling 12VDC from GPIO
Replies: 25
Views: 14002

Re: Controlling 12VDC from GPIO

The cheapest way to drive a 12V solenoid or relay is to use a MOSFET. This transistor has three terminals. Connect Source to GND, Gate to the GPIO pin of the Pi, Drain to one end of the coil. If you are using long wires you need to add a 24V zener diode with Anode to GND and Cathode to Drain, to pro...
by Tage
Mon May 27, 2013 1:40 am
Forum: Automation, sensing and robotics
Topic: PWM for servos, motors, and LEDs plus plus
Replies: 71
Views: 41260

Re: PWM for servos, motors, and LEDs plus plus

Thank you for sharing this work. I did not have any issues with following the instructions once I noticed there was a -hard and a -soft float version (I read the README file, finally) :) I am working on a remotely controlled wifi camera using the Pi with camera module and a number of servos to move ...
by Tage
Fri May 24, 2013 2:14 pm
Forum: Automation, sensing and robotics
Topic: Battery Power
Replies: 12
Views: 4721

Re: Battery Power

Try to google LiFePO4 and you will find a number of battery packs for sale and also single cells that you can combine into packs of your own design. Lithium Iron Phosphate cells have 3.2V per cell which is a bit lower than the standard "lithium" cells used in cellphones and laptops, but they are muc...
by Tage
Fri May 24, 2013 2:49 am
Forum: Automation, sensing and robotics
Topic: Resistor for transistor on GPIO pin?
Replies: 5
Views: 1330

Re: Resistor for transistor on GPIO pin?

The advice is correct. You should be able to use this MOSFET up to about 6A current at room temperature. This is how you calculate: Rdson at room temp is 0.022ohm at 2.5V and 0.016ohm at 4.5V, so let us guess that the resistance at 3V is 0.020ohm. Fig 4 shows that you need to multiply this by a fact...

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