Hex Heverter Relay activation

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rzimpala
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Hex Heverter Relay activation

Post by rzimpala »

I'm building a circut using a binary counter, nand gates and a hex iinverter to generate a pulse at one minute intervals. It's based on the 28 LED clock project on the net and obtains clock accuracy via 60 Hz line current. IT works great, an LED pulses onece every minute. The pulse is too short to activate a 9 V relay. I've tried switching transistors and a 555 ckt with no success. Need something to increase the pulse width to a level compatible with the relay.<p>Thanks.<p>-Bob
ezpcb
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Re: Hex Heverter Relay activation

Post by ezpcb »

Hi bob,<p>I think a R-C charge circuit and a voltage comparetor will work. The pulse time could be adjusted by the trigger voltage of the comparetor.<p>Mike
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rstofer
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Re: Hex Heverter Relay activation

Post by rstofer »

I would use a one-shot like a 74LS121 and then run the output through the transistor that drives the relay.
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philba
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Re: Hex Heverter Relay activation

Post by philba »

What you want is a pulse stretcher. A single 555 in one shot mode triggered by a short pulse won't work for this because the trigger pulse needs to be as long or longer than the output pulse. <p>However, why not just use an inverter (got a spare?) to flip the trigger pulse to the one shot 555? You do get a short delay (width of the pulse), I don't know if that's an issue.<p>Phil
bridgen
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Re: Hex Heverter Relay activation

Post by bridgen »

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>A single 555 in one shot mode triggered by a short pulse won't work for this because the trigger pulse needs to be as long or longer than the output pulse.<hr></blockquote>
No it doesn't, and it can't be longer than the pulse.
It needs to be of shorter duration than the required pulse length.
The 555 is ideal for this application. Note that the trigger pulse should be negative-going.<p>If rzimpala can describe the pulse he was trying to trigger the 555 from and the pulse length he requires, I'm sure we can sort it out.<p>[ May 19, 2005: Message edited by: David Bridgen ]</p>
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philba
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Re: Hex Heverter Relay activation

Post by philba »

sorry, I said that wrong. the 555 trigger pulse a is negative edge - ie transition from positive to negative to trigger. The trigger voltage needs to be high for at least as long as the output pulse. If you have a positive pulse then the timer output is limited by the pulse width. Assuming you are using a positive pulse, an inverter would set up the correct situation.
Gorgon
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Re: Hex Heverter Relay activation

Post by Gorgon »

Hi Bob,
You said the clock was made from discrete logic, counters and gates.
If you just clock a 74hc74 flip-flop with the 60sec pulse, and reset it with one of the earlier counter outputs, you can decide the exact length of your relay activation.<p>And it will be the same each minute! :) <p>TOK
Gorgon the Caretaker - Character in a childrens TV-show from 1968. ;)
bridgen
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Re: Hex Heverter Relay activation

Post by bridgen »

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by philba:
The trigger voltage needs to be high for at least as long as the output pulse.<hr></blockquote>
No it doesn't. It needs only a brief negative-going pulse.
<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>If you have a positive pulse then the timer output is limited by the pulse width<hr></blockquote>
No. With a positive pulse the timer will trigger at its end, on its negative-going transition.<p>[ May 20, 2005: Message edited by: David Bridgen ]</p>
Will
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Re: Hex Heverter Relay activation

Post by Will »

Can't you just use a standard monostable ?
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Michael J
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Re: Hex Heverter Relay activation

Post by Michael J »

Rzimpala,
You sholud look at the project Universal Relay
Nuts and Volts APR 2005.<p>It will operate a relay using any voltage in the
supply range, for that matter anything you can think of, transducer, mic, sensor.<p>You would need to get a suitable 5 or 6 volt
DPDT relay, they are available in that configuration. (article describes 12 v operation mainly,,, but will run from 5 to 24VDC with the
correct relay)<p>The relay can be made to self latch from a single
pulse also.<p>Any further questions I can answer, as I wrote
the article.
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philba
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Re: Hex Heverter Relay activation

Post by philba »

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by David Bridgen:

No. With a positive pulse the timer will trigger at its end, on its negative-going transition.<p>[ May 20, 2005: Message edited by: David Bridgen ]
<hr></blockquote><p>I dont think you understood. the pulse can't be a short postive pulse (low, high biefly and then low). it needs to be high for some period before going low. I've run into this before and even have a SPICE simulation of it. Try it. Watch the cap charge up, you'll see.
dyarker
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Re: Hex Heverter Relay activation

Post by dyarker »

A 555 and driver transistor for the relay coil are the correct parts. How long should the relay be on each minute? What are the values of 555 timing resistors and capacitor?<p>A schematic drawing would really help. At least a word description of the connections.<p>After rereading your post a dozen times, it seems just barely possible that you mean pulse amplitude instead of width. If you are using a 5V supply for the logic, connected one end of relay coil to 555 out, and other end of relay coil to common, then the relay will never activate.<p>More info needed.
Dale Y
bridgen
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Re: Hex Heverter Relay activation

Post by bridgen »

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by philba:
it needs to be high for some period before going low. I've run into this before .... Try it. <hr></blockquote>
Ok philba, will do.<p>[ May 22, 2005: Message edited by: David Bridgen ]</p>
rzimpala
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Re: Hex Heverter Relay activation

Post by rzimpala »

Thanks Everyone. Currently the input signal to the 555 remains low for approx 59 seconds. At one minute intervals there is a very fast positve (high) pulse. the duration is so short that it cannot be captured on a DVM. However if you connect an LED to the input you can visibly see the quick blink. Also this fast pulse is not enough to energize the 7-9 v relay. I belive that the siggestion of using an inverter will be very helpful. I need the relay to energize for 1-2 seconds which will ultimately pull the minut hands forward one minute on an old Sperti Faraday school clock system. This will enable an evaluation of the state of the mechanical clock movements to possibly justify purchasing a 365 day event timer for permanent install. Any suggestions on R C values for the 1-2 second pulse duration? Again Thanks to everyone this board is great!
dyarker
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Re: Hex Heverter Relay activation

Post by dyarker »

Yes, definitely use an inverter to trigger the 555.<p>From the graph on the spec sheet it looks like 20uF and Ra + Rb = 100K Ohms is about what you want. Read the text to calculate closer with standard caps and resistors.<p>My favorite place for spec sheets is [url=http://www.digikey.com,]www.digikey.com,[/url] search on part number, select one, select tech data, select manufacturer data.
Dale Y
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