Help with switching circuit

[GantryG]

Hi all- It's amazing how helpful the experts on this forum are.

I need help, myself, with the following problem:

I have an RC plane application where I need a circuit that just has one input and one output. I need the circuit to be able to switch on (or source/sink) a load after waiting two seconds after the input goes low, and then when the input goes high, have it wait for seven seconds, then switch off the load. If the input goes high during the two-second timeout, then the timer should reset and wait for the input to go low again. The seven second timeout should not be interruptable, ideally.

The supply voltage and the voltage switched is 3.7-4.2 volts (one lipo cell.) The switched load is approximately 160mA.

It seems easy enough to do this with a PIC, but I was hoping to avoid using a programmed chip. It seems like it could be done with some NAND gates and resistor/cap network, or a 555 or 556 chip in some combination, but I am not yet knowledgeable enough to be able to design a circuit like that myself. The fewer the components, the better. I was hoping that someone had a sample circuit that does much the same thing, or knew where one was...

Suggestions?

 

[Nigel Goodwin]

As it's for a plane you want small, and you want light - go for a PIC, by far the easiest way to do what you want, and it will be tiny and weigh almost nothing (particularly if you use a 10F series SM chip)

 

[Sceadwian]

Getting useful timeing control and reliabilty out of discrete logic is not going to happen. The nice thing about MCU's is once you dive into them and get used to them and realize that programming them is not like ripping your own arm out, the amount of things you can do with them is absolutly incredible. The same code that does 1 input and 1 output for a simple switch could be modified to do something like crow mixing, or exponentials, or multi channel perfectly synchronized outputs from a single input. Learn enough and you can do more with a couple MCU's than you can with the 1000+ dollar RC controllers that are out there on the market. One of Futaba's recent models actually runs a modified version of Windows CE <gag> Talk about overkill and under featured.

 

[GantryG]

OK guys, I'll go for it. If you say that it is the way to go, then so be it. I always wanted to get into programming mcu's anyway, and I know that they are great theses days.

Thanks alot, Nigel and Sceadwian!

 

[Sceadwian]

Just a last minute plug, but before you decide to use PIC's give a good look at AVR micro controllers from Atmel. They have their advantages over PICS (the primary one being higher MIPS per MHZ on the native clock. Most, ?all? pics divide the driven clock by 4 for the system clock due to internal instruction timing.
An AVR running at 20mhz can time square wave pulses at a resolution of 50ns's which for an RC signals means you have 14 significant bits. That's better than even the best commercially available RC equipment goes.

 

[GantryG]

OK, thanks alot Sceadwian. I will check it out.

 

[Nigel Goodwin]

Just a last minute plug, but before you decide to use PIC's give a good look at AVR micro controllers from Atmel. They have their advantages over PICS (the primary one being higher MIPS per MHZ on the native clock. Most, ?all? pics divide the driven clock by 4 for the system clock due to internal instruction timing.

The 18F series PIC's are the same, using a similar clock multiplying PLL. You also need to consider that many AVR instructions require more than a single instruction cycle to operate - most PIC instructions are single cycle.

An AVR running at 20mhz can time square wave pulses at a resolution of 50ns's which for an RC signals means you have 14 significant bits. That's better than even the best commercially available RC equipment goes.

And completely pointless, as the radio control system doesn't have anywhere near that resolution, as servo's don't - even a PIC running at only 4Mhz (1uS per instruction) is more than fast enough for the resolution available.

 

[gramo]

PIC's and PIC Basic FTW!