Split switchable Four & Eight Hour timer

Hi @traintester,

   It is several decades since I last used a 555 timer, but I think you are correct in suspecting it will not be satisfactory. If you only wanted a very 'rough' timing interval of say 8 hours, and could tolerate an error margin of say an hour or so, then with care, it might be possible, but I suspect a 'precise' interval of 8 hours would be challenging!

By contrast, microcontrollers can have very precise clocks, and simple programmes that count convenient units of time, like seconds, can readily be used to produce accurately timed 'pulses'. The ubiquitous 'blink' programme that is often used as a test/introduction for Arduino and similar systems, provides the basis of a timed pulse of light from an LED. This could be readily adjusted for timing periods of 4/8 hours, instead of a second or two. It also shows the wiring needed for a simple low current LED.

Of course, some LEDs, such as those used for room lighting, involve higher currents and voltages, that requires extra circuitry to interface with microcontrollers.

Bill (@dronebot-workshop) has shown several simple examples in his excellent videos, plus there are other reputable sources, including Random Nerd Tutorials ( https://randomnerdtutorials.com) and Paul McWhorter ( https://toptechboy.com/), so there is plenty of good quality reference videos and blogs available, demonstrating Arduinos, etc. performing such functions. 

In addition, a more detailed description of what you wish to achieve in a follow up question may provoke a more specific answer to how you could proceed.

Best wishes and good luck with your project, Dave

@davee  Thanks Dave for you reply, I have been doing some more searching on the internet and by chance found a really useful site and one of the items provided a great variation of differing projects including a repeating timer.

Here is one of the linked files that provides a design to satisfy my needs.

This Document covers a simulation of Ron J's  simulation but you will fine links to his other designs 

https://ronj.eu5.org/rt8x.html

Once again thanks for your assistance

Fred

Hi Fred @traintester,

  Thank you for your kind reply.

  I had a quick look at the circuit reference you provided, which for me was flash back to the same era as my former acquaintance with the 555 timer, and by coincidence, whilst I was typing my first reply to you, I momentarily considered mentioning the possibility of using a 555 timer as an oscillator with a period of (say) 10 seconds, combined with a counter, effectively replicating the circuit you found. This led my mind to wander in the usual manner when considering the 'best' choices for a new design.

Previously, I dismissed the idea of using a 555 oscillator and counter for two main reasons:

1. It used a lot of components which would need to be sourced and hand-wired, compared to buying a low-cost microcontroller board, ready wired, etc.
2. Using a 555 timer as an oscillator means the timing is controlled by resistor, capacitors, etc., which are typically low tolerance and subject to changes of temperature, supply voltage and so on, making repeatable, accurate timing a challenge. By contrast, many microcontrollers have crystal controlled clocks.

I would expect the circuit you found would be subject to similar disadvantages, with the simple oscillator based on two inverter gates, probably being even more vulnerable to temperature and voltage changes than the 555 timer.

Of course, the simulated circuit will not suffer from oscillator drift issues, and perhaps the drift of the 'real' system will be small enough to not be a problem for your application. These are typical considerations when designing a new system.

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Another sub-discussion could mention that the circuit you referenced uses a relay to drive the external circuit, which you indicated would be an LED for your application. This approach could also be adopted with a microcontroller board, allowing the "LED" to be a higher powered device, possibly powered by a separate higher voltage source. However, if the LED was a small, low voltage and current device, then it could be driven by the microcontroller directly or via a transistor buffer, similar to Q1 in the simulator circuit.

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As someone who has personally watched the progression of technology from valve (tube) technology, through discrete transistors, then specific application integrated circuits like the 555 and the CD4060, and into microprocessors and microcontrollers, it sometimes feels crazy to use a device with vastly more capability than the computing device built for the lunar landing, as a simple timer function to switch a light on and off, but in reality, it is typically the cheapest, simplest and most reliable solution. Of course, it often means considering software as well hardware, but in simple cases (at least), that can usually be achieved.

Of course, your criteria for design decisions will differ from mine, and I would not be surprised if you came to a different conclusion. The differences make life interesting!

Good luck and best wishes, Dave