74LS05 driving Relay

Kyle Winters · 2005-01-26T14:41:43+00:00

Hello everyone,I apologise in advance for my lack of knowledge. I am completely self-taught in electronics so might be missing a basic concept here...I have a 74LS05 IC (Hex Open Collector Inverter) that I want to use to drive a motor (via a relay). I tested it first with a LED and it worked fine (after I realised I needed to put a 3K9 (pull-up?) resistor to the +5V line to get it going).Now when I attach the relay in place of the LED, I don't get enough voltage to fire the 5V relay.I will try a diagram for when it worked with the LED...Output of 74LS05????????????????>?????? ?Vx LED ? ? ? > ? GND ? ????? +5VNow, when I try it with the relay in place of the LED, I don't get enough voltage. I tried various values for R1 - the results were:R1 Vx100R 2.90 V820R 0.70 V3K9 0.16 V22K 0.03 VNow, I'm thinking if I go any lower than 100R, the voltage at Vx will permanently be enough to switch the relay, so I'm a bit lost - I don't think my theory is correct.In case you need it, the resistance across the relay coil is 120R.If anyone can point me in the right direction, I would very much appreciate it. Even better, if someone can tell me why the LS05 needs this resistor at all, maybe I would understand it a bit better.By the way, I am testing this with nothing attached to the relay - I just wanted to see it switch before I loaded it. Will I need to allow for the load on the relay in the solution to this problem? I'm thinking not because they are seperate circuits anyway, aren't they?Thanks in advance,Kyle.

Jan 26, 2005 24 Replies

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Kyle Winters 21 years ago

Many thanks to everyone who replied to my cries for assistance. Fred, I followed this diagram but I still only get 2.2V across the relay. Unfortunately, I'm so far out of my depth, I wouldn't have a clue as to what to try changing to increase the voltage. Any suggestions?

Thanks again...

(diagram reproduced for clarity)

5V | 1N914 +-------+---------------+--|----| o------+--|>|----+---| 2N2222A | |/ | |\ +---- | 74LS05 / e ON | 1K | | / | | \ | | | | GND----+-----------------+-----+

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John Woodgate 21 years ago

I read in sci.electronics.design that Kyle Winters wrote (in ) about '74LS05 driving Relay', on Sat, 29 Jan 2005:

Disconnect the 7405 output pin from the junction of the top 1 kohm and the diode. Now the transistor must be hard on if it's not damaged. What voltage do you get across the relay in that condition?

You haven't got the emitter and collector reversed, by any chance, have you?

Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory. The bad news is that everything is prohibited. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

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Frank Bemelman 21 years ago

"Kyle Winters" schreef in bericht news:VzJKd.139752$ snipped-for-privacy@news-server.bigpond.net.au...

what

Seems impossible. Are you feeding the LS05 input with 60Hz noise or something? Connect the input to ground, and measure again. Also measure the other voltages, both sides of the diode and the power supply.

Thanks, Frank. (remove 'q' and 'invalid' when replying by email)

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Fred Bloggs 21 years ago

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Bradley1234 21 years ago

okay that sounds good

see

wouldnt

but that doesnt seem right. There is the work function where a charge builds up in the air so the voltage can conduct, since it never just "jumps" across an arc, it would be a fixed amount of energy/wattage but divides up with a high voltage, low current across the arc (or spark)

There is some chart I cant find now but shows the voltage relative to air pressure and gap width, the type of metal used has some effect, but it seems by guessing that a 3V supply making a spark/arc would have to be hitting around 100 v

Its not the same but a camera battery at 1.5 v or whatever is used to fuel the 1000's of volts needed for a flash

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