Which way, A MC? or Simple Transistor Circuit?

Other Hardware General Electronics
1

I'm in two minds here, using an attiny85 or creating a simple circuit, filling a capcitor, attaching it to a mosfet, then use something stupidly high like a 10million ohm resistor to ground....

I'm sick and tired of having to tell people to switch off the electric heaters in this house (they switch them on, they stay on forever until muggins here has to switch them off), so i'm going to remove their ability to switch it on and LEAVE it on..... i want it to cut out after an hour or so, my concern with the capacitor idea is the length of time i'm going to get before it bleeds to ground, my aim is around an hour or so, i guess I could use 2 transistors and charge and even larger capacitor or should i just stick with the attiny85?

2

How are two transistors going to stop the heater being left on?

3

Two solutions spring to mind:

  1. ATTiny85 driving an SSR directly, or driving a mechanical relay through a transistor.

  2. 555 timer IC and a small supercapacitor (e.g. 0.1F and about 33K), driving an SSR or a mechanical relay directly.

Either way, you will need to provide a power supply for the electronics.

4

The transistors don't directly stop the heater being left on, the transistors switch on and stay on while the voltage in the capacitors stay highish, while they are on it's because the capacitor's have not yet drained thus giving a high out, that high out will drive a relay (which keeps the heater on)... i'm worried i'm not going to get longer than an hour or so with this method, i may have to switch to another method maybe a binary counter ic....

5

Once you are looking at binary counter ICs, IMO the ATtiny is a simpler solution of you have the means to program it. However, it shouldn't be difficult to achieve a 1-hour timeout using a capacitor, if you are not fussy about the exact time. The reason I suggested a 555 is that it integrates all the components you need, and can drive a relay directly. On reflection, I think a LMC555 (CMOS 555) with a 1uF capacitor (not aluminium electrolytic) and 3.3M resistor would be more suitable than a normal 555 and supercapacitor. You could run it from 12V, then it should be able to drive a 12V relay coil at up to 40mA directly.

6

A tiny85 is the simplest method, plus you can fiddle with the timing afterwards (make it more or less).

7

A microcontroller has a clock, so why would you need analog electronics to set a time delay when
you can do it for free in software?

8

MarkT:
A microcontroller has a clock, so why would you need analog electronics to set a time delay when
you can do it for free in software?

The choice being discussed is between using analog electronics to create a time delay, or using a microcontroller to do the timing. Nobody has suggested using a combination of both.

The reasons I think a 555-based circuit may be worth considering are that it can use an unregulated power supply, and it can drive a mechanical relay directly. But the ATtiny-based solution is more flexible.