Using bright LEDs with timers for deaf people

Thread Starter

onlyonce18

Joined May 8, 2016
46
Hi, I'm trying to modify a wireless doorbell set so my father (he is deaf) will "see" when someone is at the door. I thought a wireless doorbell would be easy to modify and use it as a visual alert, besides being "portable" as he can take it with him (will be using a transformer as a power source, but still is portable). Not using batteries because I'm not confident he will notice if the batteries are with no energy left.

The wireless doorbell set has two units, the emitter of the radio signal and the receiver. The receiver has a small speaker and an LED that blinks twice when someone activates the emitter.
I am trying to replace the receiver's LED with an optocoupler and use it as a switch to start two timers, the first timer will be "on" for around 2 minutes, and the second timer will flash some bright LEDs during those 2 minutes. Power source is a 2A 5V DC out transformer. The bright LEDs use 450 mA at 5V, according to my old multimeter, so the transformer should be enough to handle that.

I built the circuit on a breadboard and it worked ok when using 2 or 3 normal LEDs, had flashing LEDs for a little more than 2 minutes. However when connecting bright LEDs they turn on only at the time I push the button, after that they turn off. I suspect this is because the sudden current demand of the bright LEDs are resetting or powering off the 555 ICs, but I have no tools to test that (only have an old multimeter).

Tried adding another 5V DC out transformer at the mosfet side, creating something like a low and high circuits (disconnected the bright LEDs from the positive side of the first transformer, connected the negative side of second to "ground" and the positive side to the bright LEDs). In this case it worked ok, the second transformer was powering the bright LEDs and the timers worked ok.

Below is the circuit I tried (got most of the parts from the internet and made a few changes). When the test work with the bright LEDs I will replace the push button and resistor at the optocoupler and connect it to the cables of the receiver's LED.

What could be the cause of this behaviour (brighter LEDs not flashing but normal LEDs do) and how can I correct it?

Thank you for your help and comments.


circuit.png
 

blocco a spirale

Joined Jun 18, 2008
1,546
I don't see a current limiting resistor for the LEDs, But at the same time 5V is a little low for 3 LEDs in series (depending on colour).

Connecting the LEDs in parallel, each with their own resistor would be a better arrangement at this voltage.
 

ScottWang

Joined Aug 23, 2012
6,963
In this application seems no needs to add that photocoupler, and in most of the cases when we using the photocoupler always apply two different power supplies.
 

AlbertHall

Joined Jun 4, 2014
10,558
Your circuit shows no decoupling capacitors on the supply - try a few hundred microfarads across the supply. You should also have 10nF capacitors to ground on pin 5 of the '555s. If there is still a problem then you may need a resistor between the main supply and the '555 power supply pins, again with decoupling on the '555 supply pins.
 

Sitara

Joined May 2, 2014
57
I second Ericgibbs suggestion (post #2). Vibrator motors are cheap, widely available & are ideal for alerting the deaf:

http://www.ebay.co.uk/sch/i.html?_f...rator+motor.TRS0&_nkw=vibrator+motor&_sacat=0

http://www.ebay.co.uk/itm/X2-Two-4x...900521?hash=item1c712da2e9:g:h74AAOSw8w1X7S4m

A battery powered (yes batteries, but using rechargeable Li-Ion, with a circuit which senses if the battery voltage falls below a given threshold & then activates the vibrator to vibrate in a distinctive pattern, indicating that the battery needs recharging - use the same idea if you need the user to replace normal expendable batteries), pocket-sized unit with vibratory alert would do the job more easily and with less hassle, in all ambient lighting conditions (including bright sunlight).

You can also use a normal, flashing LED to indicate that the battery needs recharging/replacing. That would use less juice than the vibrator, and would be as effective, provided of course that the LED is sited where it is visible even when the unit is in-pocket.
 

Thread Starter

onlyonce18

Joined May 8, 2016
46

Thread Starter

onlyonce18

Joined May 8, 2016
46
I don't see a current limiting resistor for the LEDs, But at the same time 5V is a little low for 3 LEDs in series (depending on colour).

Connecting the LEDs in parallel, each with their own resistor would be a better arrangement at this voltage.
In this application seems no needs to add that photocoupler, and in most of the cases when we using the photocoupler always apply two different power supplies.
I thought about the photocoupler as an easy way to connect the receiver unit to the circuit, instead of using another parts. The receiver unit lits a small LED when activated, instead the LED I will connect the optocoupler and use its other side as the "push button" to start the timers.
Will be easier to make the circuit with only four legs to connect.
I'm using the LEDs in series because I'm using spare parts from some broken equipments and I had the bright LEDs already mounted on a plastic piece that can be attached to the receiver unit without complications.
If needed I will break the plastic piece and make a parallel connection, but how would changing the connections fix the problem?
 

Thread Starter

onlyonce18

Joined May 8, 2016
46
Your circuit shows no decoupling capacitors on the supply - try a few hundred microfarads across the supply. You should also have 10nF capacitors to ground on pin 5 of the '555s. If there is still a problem then you may need a resistor between the main supply and the '555 power supply pins, again with decoupling on the '555 supply pins.

I will try this, thanks
 

Thread Starter

onlyonce18

Joined May 8, 2016
46
I second Ericgibbs suggestion (post #2). Vibrator motors are cheap, widely available & are ideal for alerting the deaf:

http://www.ebay.co.uk/sch/i.html?_f...rator+motor.TRS0&_nkw=vibrator+motor&_sacat=0

http://www.ebay.co.uk/itm/X2-Two-4x...900521?hash=item1c712da2e9:g:h74AAOSw8w1X7S4m

A battery powered (yes batteries, but using rechargeable Li-Ion, with a circuit which senses if the battery voltage falls below a given threshold & then activates the vibrator to vibrate in a distinctive pattern, indicating that the battery needs recharging - use the same idea if you need the user to replace normal expendable batteries), pocket-sized unit with vibratory alert would do the job more easily and with less hassle, in all ambient lighting conditions (including bright sunlight).

You can also use a normal, flashing LED to indicate that the battery needs recharging/replacing. That would use less juice than the vibrator, and would be as effective, provided of course that the LED is sited where it is visible even when the unit is in-pocket.

Please see my answer to ericgibbs. Your ideas with the vibrator patterns are very interesting but way out of my current knowledge!
 

ScottWang

Joined Aug 23, 2012
6,963
I thought about the photocoupler as an easy way to connect the receiver unit to the circuit, instead of using another parts. The receiver unit lits a small LED when activated, instead the LED I will connect the optocoupler and use its other side as the "push button" to start the timers.
If the unit of photocoupler as some kind of remote control then it's a good idea.
 

Thread Starter

onlyonce18

Joined May 8, 2016
46
Your circuit shows no decoupling capacitors on the supply - try a few hundred microfarads across the supply. You should also have 10nF capacitors to ground on pin 5 of the '555s. If there is still a problem then you may need a resistor between the main supply and the '555 power supply pins, again with decoupling on the '555 supply pins.
A 220uF capacitor between pins 1 and 8 of the first 555 solved the problem, as you suggested. The 10nF capacitors on pin 5 didn't make any difference. I will leave them as a precaution, and add one more 220uF capacitor at the other 555.

Thank you very much!
 

Bernard

Joined Aug 7, 2008
5,686
The 2N7000 is not logic level & may not fully turn on @ 5 V.
I do not have a good substitute but I use a surface mount FDD6530A, 20V, 21A, gate +_ 8V, when a LL FET is needed.
 
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Thread Starter

onlyonce18

Joined May 8, 2016
46
The 2N7000 is not logic level & may not fully turn on @ 5 V.
I do not have a good substitute but I use a surface mount FDD6530A, 20V, 21A, gate +_ 8V, when a LL FET is needed.
Yes, you are correct. I checked the 2N7000 datasheet, and at 5V it allows around 350mA with 10V between D and S, no more data at 5V DS.

Fortunatelly is working, but I should look for a suitable transistor.

Thanks!
 

Thread Starter

onlyonce18

Joined May 8, 2016
46
Thank you all for your help and comments, just one last question. I already built the circuit and it is working, but I noticed that having the blinking LEDs for 2 minutes is too much if you already saw it, so I added a bush button to stop the first timer.

Not sure if it is the right place to put the push button, but it is working (please see schematics below).

I thought the push button there would result in two things, stop the timer because the voltage at pin 6 would be high enough, and charge the capacitor C2.

Please let me know if there is a better place, and thanks again!

schematic.png
 

AlbertHall

Joined Jun 4, 2014
10,558
My first choice would have been to put the switch on the reset for that '555 but that would need an extra resistor. Shorting the timing capacitor will work and I see no downside - plus as you say, it works :)
 

Bernard

Joined Aug 7, 2008
5,686
Might not be a good idea to have a direct connection from + 5 V to pin 7, discharge. I do not know current rating of p7, thought I saw a rating of 200 mA a long time ago, but I think it must really be constant current limited.
 

AlbertHall

Joined Jun 4, 2014
10,558
Might not be a good idea to have a direct connection from + 5 V to pin 7, discharge. I do not know current rating of p7, thought I saw a rating of 200 mA a long time ago, but I think it must really be constant current limited.
Or, better, use the reset pin to do the reset. Connect the reset pin to the supply with a resistor, say 10k, and then connect SW1 from the reset pin to ground.
 
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