Hi guys,

I made up Hero999's circuit on a stripboard, but the relay doesn't appear to be doing anything. Is pin 3 on the 555 timer capable of driving the relay?
I have tried removing the diode between the coil of the relay & replaced it with a resistor & LED. This was so that I would be able to see the led illuminate when the relay was switched over & it should then turn off when the relay switches back. However the LED wouldn't illuminate at all.

I even purchased the astable from Maplin, but it appeared that some parts were missing from the kit! (an LED was in the kit, but didn't connect to anything due to a missing resistor). I have tried using the relay supplied in this kit on my stripboard, but it too doesn't seem to do anything.

I have redesigned the stripboard in case i missed something, but I still cant seem to get this to work.

Any help would be greatly apreciated.

 

If you build my circuit, make sure the diodes, polarised capacitors and IC are connected the right way round.

Never remove the diode in parallel with the relay, it's there to protect if from the high voltage pulse generated when the relay turns off.

It sounds like you might've got the protection diode backwards, did the 555 get hot?

What current does the relay draw?

The maximum a 555 can supply is 200mA.

Sometimes the -0.6V generated by the relay coil can interfer with the 555, adding a diode in series with the relay and protection diode might help.

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I may have got one of the diodes around the wrong way as the 555 did get hot.

However I rebuilt the circuit completely & still the relay wouldn't fire.

I'm starting again! I've tested the relays & one of them was faulty. I'm going to try & adding the 3rd diode & will keep my fingers X'd.

Thanks

Attached Images

circuit.jpg (35.0 KB, 12 views)

 

As your 555 got hot it's probably been cooked as a huge current flowed though pin 3 and the protection diode.

I recommend replacing both the 555 and protection diode when you rebuild the circuit and it'll probably work.

Also the capacitor from pin 5 needs to be about 10nF, not 10µF.

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Is there a good way of testing the 555? I tried using the one from the maplin kit, but the circuit's still not doing anything. It's not getting hot this time.
The capacitor on pin 5 in 10nf (just a typo on the diagram).

This is the 3rd attempt & I'm getting pretty frustrated with what should be a simple circuit.

I don't suppose anyone wants to make this up for me & I'll pay for components, materials / time etc??

Cheers

 

Do you have a multimeter?

Measure the supply voltage, does it read 12V?

What are you using to power thing with?

Is it an unregulated 12V mains power brick?

Some of these can output voltages as high as 20V with a light load, if so your 555 only has an absloute maximum voltage of 16V and might be destroyed.

Measure the voltage on the capacitor, does it reach 0V pretty quickly when the power is removed and charge up more slowly when the power is applied?

Are you using the correct resistor can capacitor values? Measure the resistor with a multimeter to make sure. I doubt you're using 500k which is a non-standard value, unless it's a pot.

Your origional post said you require a delay of 10 seconds, the components values you have chosen only give about 5 seconds.

Try replacing the capacitor with 100µF and the resistor with 47k or 100k depending on whether you want 5 or 10 seconds. Somtimes the leakage capacitor current can be too high so it never charges though the high value resistor.

If I remember rightly the 555 supplied in the Maplin kit is a TS555 and can only drive 100mA; are you sure your relay uses less than this?

You can easilly test a 555 timer by building an astable that flashes an LED but the circuit is just as complicated as this one which you are having trouble building so I wouldn't recommend it.

Put the circuit and all your components away and return to this problem another day when you've rested.

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The multmeter shows the cap is charging / discharging. I've checked the circuit several times & the 555 just keeps getting hot, so I guess it's well & truly cooked.

I have the relay protection the correct way around, anode to negative & cathode connected to pin3. The other diode is connected with the anode connected to pin2 & cathode to positive. I have also added the additional diode that you mentioned on pin3 anode to pin3 cathode to relay & the cathode of the first diode I mentioned.

The input voltage is from the cigarette lighter in the car, but I have been testing the last 2 circuit revisions with a PP3 9v battery.

Im using a 5k6 resistor, but the delay isn't too important, somewhere between 5-10 seconds is fine.

I'm going to reorder the parts from maplin & start from scratch.
I have posted the list of components here, can you confirm all the parts are correct.

Cheers!

Attached Images

parts.jpg (65.9 KB, 9 views)

 

The delay is important, it at least needs to be long enough to turn the relay on and for you to see it.

Delay = 1.1RC

Let's calculate the delay for your circuit:
R = 5k6 = 5600
C = 10µ
Delay = 0.00001

Delay = 1.1×5600×0.00001 = 0.0616s = 6.16ms

A delay of 6.16ms will be far be too short to turn the relay on let alone for you to notice.

The ceramic capacitor and 4k7 resistor you're intending to order will give an even shorter delay, just 0.000517 or 517µs which is even worse.

Start off with 100µF and 100k for 11s.

If you want a variable delay, then go for a 100k variable resistor with a 1k fixed resistor in series to give a delay from 0.11s to 11.1s but only do that once you've got the 11s delay working.

Where abouts do you live?

I live near Bedford.

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I've removed the ceramic capacitor & added 1*100K pot, 1*100K resistor, 1*1k resistor & 1*100µF capacitor.

I live in Tamworth (just north of Brum), but travel around a fair bit for work.

 

You don't need the 100k if you're using a 100k pot.

Think about it, it will give a resistance range of 101k to 201k, you want 1k to 101k for 0.1s to 11s, not 10s to 20s but it's up to you.

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I know. I thought I would attempt to use the 100k resistor first & perhaps use the Pot once the circuit is working. I should have chosen a 10k - 100K Pot, though.

Thanks for taking the time to explain this all. It is really apreciated. I did study electronics several years ago & was actually quite good at it (having even designed a circuit that tracked the strongest source of sunlight to keep a solar panel generating the most amount of power available. It also checked to see if it was dark, as there was no point in wasting power looking for sunlight if there was none!), but it's amazing what you forget when you don't use your knowledge.

 

Maybe I've missed the jist of the thread here, but shouldn't pin 7 be connected to pin 6 too? I notice you have Germanium diodes in your parts list. A 1N4148 would work OK, and a lot cheaper

 

On my circuit simulator it seems to work without pin 7 being connected.

Thanks for the tip about the diodes, but I've already put the order in. Apart from being cheaper they should work just as well. (I was thinking higher cost = better reliability / performance) - even if that's not the case!

 

NP Craigey,
the only difference is the voltage drop approx 0.3v on germanium and 0.6-7 on silicon)
I rarely use 555 timers, so I may be wrong.
I have a pile of 555's at work somewhere, I'll mock your circuit up, if I get a chance.

 

No, pin 7 should be left unconnected.

That's ture, the voltage drop doesn't make any difference here. The diodes are just used for protection and discharging a capacitor so using expensive ones is a waste of money.

They will work.

However don't just assume that a more expensive part will be better. In this case you chose a more expensive part and it doesn't matter but in some cases it could be worse. For example suppose this was a voltage regulator in a power supply and you subsituted the cheap power transistor for an expensive lower powerr rating RF transistor: in this case it would be worse and probably burn out because although it might be more expensive, it still has a lower current rating.

The moral of the story is, always check the datasheets of the components you use rather than making assumptions based on price or the package design.

just an after thought but what's the current rating of the diodes?

Germanium diodes are low current devices, I'm concerned that the one in series with the relay won't be able to take the current.

If you wanted to spend more moner, you could've bought a more expensive 555 like the ICM7555 or the TS555, they will use less power and have a lower input current which is better for longer delays.

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