# 5V trigger by Light - Need comments & Suggestions or Corrected

#### nizee

Joined Apr 29, 2008
12
I'm planning to build a circuit that sense light from an Optical output on the TV and then Trigger a 5V output and at the same time light-up the LED. If there is no light, the Trigger output will be zero volts. The active 5V will tell my other device the status on my TV if it's ON or OFF.
Please comments if i have it right or if the output really a 5V or less?
Feel free to edit if you think a better way of doing it will less parts.
Any suggestion on the part number of Photoresistor i can use, say from local store, eg. Radioshack or Fry's Electronics?

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#### SgtWookie

Joined Jul 17, 2007
22,210
The output will be somewhat less than 5v; actually somewhere around 4.4v due to the 0.6v (approximate) voltage drop across Q1's emitter.

Consider increasing R3 to 50K to 100K for more sensitivity adjustment range.

Re-calculate R2 for the proper current limiting resistance for D1.
Example: If D1's Vf (forward voltage) = 2.1 @ 30mA then
R2 = (Vsupply - Vf) / 30mA
R2 = (4.4 - 2.1) / 0.03
R2 = 2.3 / 0.03
R2 = 76.666... Ohms
The nearest standard value resistor larger than or equal to the result is 82 Ohms - always go up to the next higher value, never down.
To calculate the current flow with the new value of resistance:
I = E/R
I = 2.3/82
I = 28mA
Wattage needed for resistor:
P = EI
P = 2.3 x 0.028
P = 64.4mW
A 1/10 Watt resistor or larger would be fine.

#### nizee

Joined Apr 29, 2008
12
thank you for the reply SgtWookie.
What if i put/connect the trigger line at the Collector, instead of emitter on the transistor? Then reverse the Photoresistors with the variable resistor/resistor network.
Will i get 5V trigger (without voltage drop) if light shine on the photoresistors?

#### SgtWookie

Joined Jul 17, 2007
22,210
You would need to use a logic-level P-channel MOSFET in order to get nearly 5V out of it.

4.4v is plenty high enough for TTL-level logic; as long as you're over about 2.8V, TTL sees it as a logic high level.

Is there some reason that you need 5v out?

#### nizee

Joined Apr 29, 2008
12
Let me visit local RadioShack and buy some of this components to test out the board and see if it works.
The equipment i have say it required 5V trigger to knowledge it's "ON".
I need to assemble the circuit to see if it accept 4.4V.

#### SgtWookie

Joined Jul 17, 2007
22,210
Radio Shack has an assortment pack of five CdS photocells for under $3. They should work. Be aware, you will need to either have the photocell close to the light source, or have a way to collimate (focus) the light on the face of the photocell. You will also need to shield the photocell from other sources of light. They also have resistor assortments; somewhere around$7 and $13. It's an expensive way to start a collection, but you need to start somewhere. They also have 15 NPN (#276-1617) and 15 PNP (#276-1604) transistor assortment packs for about$2.60 each. This is the least expensive way to get transistors at RS. They usually contain 2n2222, 2n3904, 2n3906, 2n2907, and several other types. They will come in handy for this and other projects.

LEDs: They have a 20-pack assortment (#276-1622) for under $3. It's still expensive as far as LED prices go, but it's their best value. Thread Starter #### nizee Joined Apr 29, 2008 12 Radio Shack has an assortment pack of five CdS photocells for under$3. They should work. Be aware, you will need to either have the photocell close to the light source, or have a way to collimate (focus) the light on the face of the photocell. You will also need to shield the photocell from other sources of light.

They also have resistor assortments; somewhere around $7 and$13. It's an expensive way to start a collection, but you need to start somewhere.

They also have 15 NPN (#276-1617) and 15 PNP (#276-1604) transistor assortment packs for about $2.60 each. This is the least expensive way to get transistors at RS. They usually contain 2n2222, 2n3904, 2n3906, 2n2907, and several other types. They will come in handy for this and other projects. LEDs: They have a 20-pack assortment (#276-1622) for under$3. It's still expensive as far as LED prices go, but it's their best value.
I have all the components accept the photocell & variable resistors.
I'm planning to buy those Optical extention/extender plug. One end will be the optical cable coming from the TV and the other will fit/glue the photocell. So, the Optical cable light will shine directly when it's ON and darkness when the TV is OFF.
I hope my local Radioshack have both of this item i need.

thank you for your advice. I'll update my circuit when i've everything.

#### nizee

Joined Apr 29, 2008
12
I did some search online and read some material. I've seem update the circuit from the original above circuit. See attachment for latest circuit.
I assemble them on the veraboard and seem to work better then the original circuit and having 5V output. But i have not link to the other side of the trigger input device yet.

Any comments on this new circuit is apreciated? Will it be better and reliable then the original circuit?

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