# Where to put LDR and NPN Transistor in a circuit?

#### psoriasisninja

Joined Dec 7, 2015
5
Hello,
I bought some HO Scale lamp posts that come with a diagram that is pretty detailed. My son needs to make the street lights go on when it becomes dark but we are unsure if there is a specific place for the LDR and NPN Transistor (and what particular kind if we use 5 lights in a circuit) on the given diagram. Any help would be greatly appreciated! Thank you in advance!

#### AnalogKid

Joined Aug 1, 2013
8,826
Note that depending on the *total* amp current, the transistor might need a heatsink. If these are small incandescent lights, then maximum heat dissipated in the transistor comes when it has 8 V across it (and 8 V across the lights).

ak

#### djsfantasi

Joined Apr 11, 2010
7,247
Do you know the amps drawn by all five lights? Or that drawn by one light? In order to determine the other values, this needs to be known.

In order to find out the current (amps) for one light connect one lamp as in the diagram, but leave the other resistor end unconnected. Using the 10 Amp scale on your DMM (which you do have?), connect one prove to the resistor and the other probe to the power supply ground. The light should light and you'll see the current used on the meter. If it's a small value, you might be able to use a smaller scale. I'd draw a pic, but I'm on a subway.

Multiply that current by 5, and that is how much current the transistor will have to switch.

#### GopherT

Joined Nov 23, 2012
8,012
Note that depending on the *total* amp current, the transistor might need a heatsink. If these are small incandescent lights, then maximum heat dissipated in the transistor comes when it has 8 V across it (and 8 V across the lights).

ak
I assume the OP will still use the prescribed limiting resistors (see the first post - operating instructions from the manufacturer.).

There is only a switch for now, I assume minimal voltage drop across the transistor as there would be minimal voltage drop (0V) across the switch.

#### AnalogKid

Joined Aug 1, 2013
8,826
Beyond the (admittedly narrow) range of light levels that would hold the transistor in a conducting but unsaturated state, consider the placement of the LDR with respect to the lights. In the post #2 circuit, light from the street lights illuminating the LDR is a form of negative feedback. You could have a situation where the lights come on at partial brightness and sit there no matter how dark the rest of the ambient is.

ak

#### psoriasisninja

Joined Dec 7, 2015
5
Thank you all for your replies! I forgot to put it out there that I'm a mom that went in over her head when her third grade boy asked to do add a miniature model of street lights for his science project. He has a snapcircuit version of how a photo resistor works but the wiring of actual lights have got us stumped. We will be heading over to the electrical supply store today with all of your input in mind.

#### djsfantasi

Joined Apr 11, 2010
7,247
Can you share the Snapcircuit version?

#### psoriasisninja

Joined Dec 7, 2015
5
Can you share the Snapcircuit version?
Yes, here's the diagram he followed where the side switch (s1) is replaced with the photo resistor and makes noise with light activation.

#### psoriasisninja

Joined Dec 7, 2015
5
Yes, here's the diagram he followed where the side switch (s1) is replaced with the photo resistor and makes noise with light activation.

#### Attachments

• 410.5 KB Views: 12

#### GopherT

Joined Nov 23, 2012
8,012
I forgot to put it out there that I'm a mom that went in over her head when her third grade boy asked to do add a miniature model of street lights for his science project.
It's not magic, I am sure you two will be successful. I think it is great that you dug in and are willing to go to the electronic store.

If you run into difficulties, just keep asking more questions here - I'm sure someone will be around here to help as you need it. We can spoon-feed as needed.