# Binary adder with 7-seg display

#### elec_mech

Joined Nov 12, 2008
1,500
Okay, according to the datasheet (see page 2), the typical forward voltage is 1.8V with a current draw of 1mA per segment.

Will I need a separate voltage for the displays? I have 3.3v regulators.
No, the current limiting resistors will take care of the voltage difference while protecting the segments from too much current, so the 3.3V regulators are not needed.

I think the display uses 1mA (it gives descriptions based on 1mA) so 5v *.001 = 5K ohm resistor correct? Can I use a 5.1K to make the display a bit darker?
Not quite. Look at the equation I provided earlier:

R_led = (Vcc - V_led) / I_led

Assuming you are still planning to use a 5VDC supply (Vcc), the current-limiting resistor values you want to use will be:

R_led = (5VDC-1.8VDC) / 1mA = 3.2VDC / 0.001A = 3200Ω or 3.2kΩ

You could use two 1.5kΩ in series per segment to get 3kΩ which would be close enough without having to reorder more parts. If you want it dimmer, you could use a 4.7kΩ or 5.1kΩ and see if it is dim enough.

Hope this helps.

#### pillyg

Joined Sep 19, 2011
52
Ok yes it does.
So the wall wart current doesn't matter? I am still deciding on whether to use a 1.5A one or a 850 mA one.

Also, how accurate are they? I have read that the actual voltage can be 3-4 volts higher than what they say.

William

#### elec_mech

Joined Nov 12, 2008
1,500
Also, how accurate are they? I have read that the actual voltage can be 3-4 volts higher than what they say.
It depends. Most wall warts are unregulated meaning the output voltage is higher than it states, but the voltage will drop depending on the current draw.

Before delving too far into this, are you using an unregulated 5V wall wart (most 5V wall warts are regulated since they are designed for use to power digital devices) or are you planning to use a 7.5V wall wart with a 5V regulator, such as the 7805?

The current rating depends on the total current draw of your circuit. The ICs shouldn't consume much, typically LEDs and 7-segment displays are your power hogs. So, at 1mA per segment, if all of your segments are on (all showing 8), then the current draw would be 1mA per segment x 7 segments per display x 7 displays = 49mA. To be safe, I'd say a power supply of 60mA or more would be fine.

#### pillyg

Joined Sep 19, 2011
52
I will have to measure the actual voltage coming from the 2 wall warts (speaking of multimeters, is http://www.ebay.com/itm/New-VC97-39...144?pt=LH_DefaultDomain_0&hash=item588b697848 alright? I saw eevblogs review saying it was ok but I think that was for pro stuff, I just want basic stuff.)

The current on them is 850 mA and 1.5A!! I think I will need a resistor or 2 . Should I use one right off the bat to drop it to maybe 100mA then use others for the various devices?

If the supplys are 5v or less I will have to use a stronger one. Not exactly sure what I have but I know there is an 18v one which I might have to use. But I am for sure using a 7805.

#### elec_mech

Joined Nov 12, 2008
1,500
Yeah, that does look nice. The AC/DC voltage and current range is great. Looks like it has a transistor checker which is always nice. Diode check is a must in my opinion and it has that. All in all, this looks like a great meter.

The current on them is 850 mA and 1.5A!! I think I will need a resistor or 2 . Should I use one right off the bat to drop it to maybe 100mA then use others for the various devices?
No, not quite. With current, you need to supply the maximum or more than what the circuit will draw. If your supply is rated 1A but your circuit only pulls 100mA, no problem - the circuit will only pull 100mA, not 1A. If your circuit pulls 1A and your supply is only rated for 100mA, then you're going to have problems. The supply will heat up trying to provide the needed current, the voltage will drop as a result and you could have a fire hazard. So, in short, don't worry about adding resistors to drop the current to the circuit, you don't need to.

Now, with voltage, you need to make sure you keep it in within the operating range of your circuit. Your 7805 regulator will take care of this as long as your wall wart rated voltage is between 7.2 - 30VDC. Just be sure to add filter capacitors on the input and output of the regulator as suggested in the datasheet.

I'd suggest a 9VDC wall wart rated 100mA or higher. You can certainly use one with a higher voltage, but note that the bigger the voltage difference between the wall wart and the regulator and the higher the current draw, the more energy will be wasted and turned into heat requiring you to add a heat sink to your regulator and possibly requiring you to cool it with a fan. Using a 9V wall wart will allow the regulator to generate less heat than using a 18V wall wart. If you, your family or friends have an old cordless telephone they don't use anymore, most of those use a 9V, 200mA wall wart which would be perfect for your circuit. The CMOS ICs in your circuit can operate from about 5 to 15V, so you could even skip the 7805 if you wanted as long as your wall wart is rated below 15VDC - you'd just need to recalculate the values for the current-limiting resistors for the 7-segment displays.

Also note that although your wall wart may be able to supply 850mA or 1.5A, most 7805 regulators can only supply 500mA max. Some go as high as 1A or 1.5A. However, your circuit should draw less than 100mA, so no worries there.

#### pillyg

Joined Sep 19, 2011
52
Ok. That is good news.

So I can use a 5mA display or IC with a 1.5A supply? (as long as the voltage is right)

Will/can the 7805 act as a current regulator?

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#### elec_mech

Joined Nov 12, 2008
1,500
So I can use a 5mA with a 1.5A supply? (as long as the voltage is right)
Yes.

Will/can the 7805 act as a current regulator?
No, it will only act as a voltage regulator to my knowledge. You can use an LM317 regulator as a constant current source if you needed to drive several LEDs.

For this project, you're fine using the 7805 as a voltage regulator. The current-limiting resistors for the 7-segment displays are all you need in terms of current regulation.

#### pillyg

Joined Sep 19, 2011
52
Score! I found a phone power supply that is 9v 300mA. I will use that.

On a side note, have you ever ordered electronic samples?

#### elec_mech

Joined Nov 12, 2008
1,500
On a side note, have you ever ordered electronic samples?
Yes, I have on occasion.