Are there any components which will resist for say 15 to 30 seconds after which let a small current though and then start to resist again?

Thanks in advance

j.r-xrs

 

Wifes....

 

 

In a word, no.

There are components that:

- have a low resistance unless you try to draw too much current, at which point they heat up and sharply increase resistance, cutting the current off. These are typically used in the same way as fuses, for overcurrent protection.

- have a high resistance, unless too great a voltage is applied across them, in which case the resistance drops sharply. These are used as overvoltage protection, to short out voltage spikes.

If you explain a bit more about your proposed application, maybe I (or someone else) could suggest an alternative?

A.

 

Ok thanks,

I have an LED and I just want to make it flash either every 15 seconds or 30 seconds, decided by a switch. However I don't want it to be too complicated or expensive, although if that’s the only way it can be done then I'd like to have a go.

Cheers

 

The easiest way to do that is probably with a 555 timer IC - any book on electronics will explain how to wire one up to do what you're looking for.

Another option would be to build an oscillator out of a logic inverter - a 74HC14 would work well. I'll scribble a schematic if you like.

A.

 

geeez , 555timers .. remember them from my old GCSE electronics course.

damn, that was a long time ago.

Steve

 

bimetallic strip? I'm sure you can spec the timings in Maplins now

 

Excellent, thanks for the info guys.

An assortment of electronic components housed in a quality storage box intended for students following the GCSE courses in Electronics. The pack includes a wide selection of capacitors, resistors, transistors, diodes, ICs, LEDs and a stripboard for assembling projects. It is also suitable for any beginner to electronics who requires a good selection of components, as the pack represents excellent value, offering a considerable saving over individual prices.
Contents
Parts organiser box - 1
BC108 - 2
BC548 - 2
1N4148 - 4
1N4001 - 4
555 Timer IC - 2
741 Op-amp - 2
Red LED - 2
Green LED - 2
Stripboard 3939 - 1
Buzzer 6V - 1
10k Pot - 1
8-pin DIL Socket - 2
100pF Ceramic - 4
470pF Ceramic - 4
1nF Polyester Film - 4
2.2nF Polyester Film - 4
4.7nF Polyester Film - 4
10nF Polyester Film - 4
22nF Polyester Film - 4
47nF Polyester Film - 4
100nF Polyester Film - 4
220nF Polyester Film - 4
470nF Polyester Film - 4
1µF Radial Electrolytic - 4
4.7µF Radial Electrolytic - 4
10µF Radial Electrolytic - 4
47µF Radial Electrolytic - 4
100µF Radial Electrolytic - 4
220µF Radial Electrolytic - 4
470µF Radial Electrolytic - 4
Resistors - 10 of each of the following values:
10 47
100 220 330 470 680
1k 2k2 3k3 4k7 6k8
10k 22k 33k 47k 68k
100k 22k 33k 470k 680k
1M


So I would be well away with this kit?
Considering that it has to 555 Timers already, would these be the type that i could vary the time laspe of, or would just just ran at a certain time period all the time?

Cheers

 

You vary the time periods of a 555 timer by using different value resistors and capacitors on some of the pins, IIRC. You can also vary the mode (monostable, astable and bistable). 555 timer is a quality bit of kit.

Alternatively, I think you can get a LED and a 'flasher' IC integrated into the same package. Again, you will be able to vary the various timings.

Alternatively you can make a clock out of crossed NAND gates and capacitors

[Edited by carl - 1/20/2003 4:43:01 PM]

 

someone may have already mentioned this, but maplin's have projects you can buy, that do this sort of thing.

regards
BB

 

Carl, how does the NAND and capacitors clock work?

Cheers

 

Can't remember exactly, but it involves taking the output through a capacitor then dumping it back into one of the inputs. Maybe there's a Schmitt trigger in there too

Thinking about it that'll work. Connect one of the NAND gate's inputs to Vcc. Then on the output put a capacitor of suitable value, then a Schmitt trigger. Connect the output of the Schmitt trigger to the other input of the NAND gate.

Initially you have a '1' in one input, and nothing in the other (actually the gate will start up in a random state, but it works either way). So the NAND gate's output will be a '1'. You use this output to charge the capacitor up, then when it goes over the trigger threshold it will discharge and output a '1' from the Schmitt. That'll become a '1' into the input of the NAND which will then output a '0' and so on. Actually thinking about it you might need a pair of NANDs working as an RS flip-flop to make this work, as you want every discharge of the capacitor to flip the state from one to the other.

555 timer is easier

[Edited by carl - 1/20/2003 4:55:58 PM]

 

I think I'm going to buy some kind of kit from Maplins and have a play, thanks very much for all the help, I'm sure I'll be back with a whole set of questions by the end of the week.

Cheers

j.r-xrs

 

If you're going to use a 555, it'll be in astable mode. Work out the components from the panel below. Remember to connect the LED through a current limiting resistor.

 

If you want it to change between 15s and 30s low period when the switch is pressed, you'll be wanting to put in a dual pole, double throw switch (I think, looking at the diagram) to switch in a different pair of R1, C1.

Edit: actually that will alter the high period too. Probably better to just switch in a different R2 and invert the output (I'm assuming you want a long low period followed by a short high period, like a car alarm LED).

[Edited by carl - 1/20/2003 5:20:55 PM]

 

Farnell have flashing LEDs. It don't get any easier than that
And they're cheap

 

Flashing LEDs are easy to use, certainly, but they flash much faster than once every 30 sec.

Here's another suggestion which you can build using your 741 op-amp:



The 'off' time is determined by R1 x C1, and the 'on' time is determined by (R1//R2) x C1.

R5, R6 want to be the same as each other, but the value isn't critical. R3 influences both the on and off times, try making it the same as R5 and R6 to begin with.

The switch puts another resistor (R7) in parallel with R3, which should slow everything down (30 sec vs 15). Try making R7 the same as R3 to begin with and adjust as necessary.

D1 can be any small diode (eg. 1N4148), and R4 sets the LED current (suggest 5mA). C1 also affects the overall speed of the circuit. If you want the 'on' time longer than the 'off' time, turn D1 around.

Suggested values to begin with:
C1=470uF, be sure to get it the right way round!
R1=33k
R2=1k
R3, R5, R6, R7 = 10k
R4=1k (assuming you're using a 9v battery for power - the supply voltage shouldn't be too critical)

Don't blame me if it doesn't work - I just made this circuit up off the top of my head! (Have fun playing with it and trying to figure out how it's supposed to work...)

A.
(very bored electronic eng )

 

Did anything ever come of this? Which circuit did you end up using?

A.

 

I havent acted on it yet, running very short of spare time, but Ive got half term and this comes as a handy reminder, ill keep you posted.

Cheers

j.r-xrs

 

Prefer Lukes synopsis

 

Maplin also sell a "self flashing" LED. No switch, admittedly, but costs £2.50 and just needs an inline resistor. Much less hassle.