As far as a charger IC, thats why I bought those blue boards.. I figured they would take care of it. But now I'm wondering if I even need it. The lipos have protection circuitry in them that prevent over charging and low voltage cut offs.
Protection circuitry is for protection. You're not supposed to use it to control the normal charging process. Chargers use carefully determined algorithms and voltage, current and temperature sensing to charge the battery to optimise safety, performance and battery lifetime. Several manufacturers make battery charging ICs - try Maxim (
http://www.maxim-ic.com), Texas Instruments (
http://www.ti.com), Microchip (
http://ww.microchip.com) and Linear Technology (
http://www.linear.com) to start with.
Ahhh that's the big difference. I've always had trouble determining when I should use a MOSFET vs a transistor. I've just known fets are usually always used when you need a lot of current to drive motors and things and transistors are usually for things that draw less current like logic circuits. I am guessing here but I assume if you did need to vay the output current on a transistor, you'd use resistance and you'd do something like PWM for varying MOSFET output.
True, MOSFETs are most often switched, but transistors can also be used with PWM.
Major advantages of MOSFETs are: (a) the drain-source path is resistive (assuming sufficient gate bias) and its resistance can be extremely low - less than 1/1000th of an ohm, and (b) The MOSFET's gate does not draw any steady current, although MOSFETs have significant gate-source capacitance. These characteristics make them good for switching heavy DC currents, and at low switching speeds, the energy required to switch them ON and OFF can be very low.
For example, you could switch a 100A current ON and OFF with a single high-current MOSFET, controlled by a tiny little switch, with the switch controlling the voltage from a stack of four 3V lithium coin cells, and the coin cells would last practically their whole shelf life.
Transistors are quite different to drive. To keep a transistor conducting, you have to feed a steady current into the base. (This current comes out the emitter.) A transistor has a limited current gain (normally less than 1000, often around 200), so the amount of current it can control is determined by the amount of base current that the driving circuit is forcing into it, and at high collector current, the current gain can get very low - less than 10 for a big transistor at 10A upwards.
Some characteristics of transistors may make them more suited to certain circuit positions than MOSFETs. The base-emitter forward voltage of around 0.7V (at low currents, at least) is less variable than the gate-source threshold voltage of a MOSFET, which can differ by as much as 2V between different devices with the same part number. On the other hand, MOSFETs have advantages in some circuit positions too. In some circuits, either will work.
The fact that transistors will turn OFF unless actively biased by a base current can be useful; if you leave a MOSFET's gate unconnected, its voltage will float, because of the extremely high gate resistance. Even an extremely weak (high-resistance) leakage path from the gate to some other voltage in the circuit will pull the gate to that potential.
Because of their extremely high gate resistance, MOSFETs can be damaged by electrostatic discharge (ESD) to the gate, which damages the thin gate insulation. This will also happen if the limited gate voltage range (no more than +/- 30V relative to the source) is exceeded. You should use antistatic precautions when handling MOSFETs (and ICs which contain them) to prevent ESD damage.
Transistors can be damaged by reverse base-emitter voltages greater than about 5V. The base-emitter junction acts as a zener and breaks down at a reverse voltage of around 7V and even a small reverse current causes both a short-term and a long-term reduction in current gain, while a larger reverse current makes the transistor unusable.
That's not a complete summary of the differences but it covers some of the main ones.
Great. I didn't catch that in the minimal documentation on that GSM.. I did catch it has a sleep mode that pulls 1.5ma but I wasn't planning on using that.
I'm not sure what that specification means. Sleep mode isn't described anywhere. Is this its default mode when it's not communicating on the GSM network? Is it always ready to receive an incoming call or message in this mode?
The only other current consumption figures are 0.5A continuous and 2A peak, which I assume only apply when the module is transmitting. So when it's idle but ready to receive, how much current does it draw? Is this the 1.5 mA Sleep mode current? Or does it draw more than 1.5 mA when it's idle but ready?
OK and for creating sounds. I don't think I'll need any interrupts or timers (but I should try them out just to understand how to use them), I assume I can just use loops and delays to generate the frequencies I need found here
http://www.phy.mtu.edu/~suits/notefreqs.html
That depends on how you're handling the serial data reception. If your code is making a half-second beep sound using instruction loops and delays, and the GSM unit sends a string like "NO CARRIER", the first nine characters are going to be lost, because the PIC wasn't checking the USART's receive ready flag during the time they came in. The USART can only store one character plus the one it's currently receiving. So you'll lose data.
There are various approaches you can take to deal with this. As I explained in an earlier post, you can use the USART's received character interrupt to force the PIC core to deal with characters as they arrive; this will cause tiny disruptions to the sound which may or may not be noticeable. Or depending on the capabilities of the timer modules in the particular PIC you're using, you may be able to time the edges of the tone using on-chip hardware, so the PIC core can do other things while it's waiting. You could even avoid using interrupts and timer features altogether if you use cycle counting!
I'll have a look at the data sheet for the PIC16F628A and suggest something.
I will do some experimenting with the speakers. That is a bit of a ways away.. I'm still waiting on my MOSFET so I can get the booster circuit hooked up. Do you think I even need to bother with turning the booster power on and off from the pic?
I think it's probably a good idea. The booster will draw significant current even if the bell isn't being used. You're looking at a normal operating current budget in the milliamp range, and the booster's idle current could easily be 10 mA.
