LDR circuit

[philba]

Yes, I missed that part, but it was still a bit over complicated.

I would use two comparators with open collector outputs, each would have different reference voltages and some hysteresis. The LED would have three series resistors at its ground connection, I would use the comparator outputs to short the appropriate resistor when it gets too dark, I canl draw to a diagram if you don't get me.

yes, that's a good one - basically my 3rd option, if you hadn't noticed. though I think hysteresis isn't necessary.

by the way, while it's kind of silly for a mower. some autos have tri-level instrument lighting based on ambient light. it's backwards from the intent here but it makes sense - bright lighting when it's bright, middle during "tween" times and dim when it's dark.

 

[bananasiong]

compare this circuit:

and this circuit:
**broken link removed**
which one uses less power?

*is a photo resistor same as the light dependent resistor?

 

[docel]

Silly it may be, but quite a bit of applications! LG TVs use this for ambient light controlled Brightness/contrast, with an enhanced cost, ofcourse.
Also in many instrument/annunciator display lamps to save battery power during day-light hrs.
"Silly" might mean " Whatever-for in a Lawn mower?!" kind of thing.
But this thread has helped many, as I can see.....for many other applications.

 

[Nigel Goodwin]

Silly it may be, but quite a bit of applications! LG TVs use this for ambient light controlled Brightness/contrast, with an enhanced cost, ofcourse.

It's been used by a great many TV's over the years, the first I can recall were some Pye sets from the 1950's, but that's not to say they were the first?. However, it's always been a really useless feature - but it's very cheap to do!, and it's an extra gimmick - just make sure you can turn it off.

 

[dknguyen]

Yes, a photos resistor is the same thing as an LDR. (It's resistance lowers with increasing light levels, right?)

I think the circuit with the resistor, LDR, and LED use the least power just because it has less components(plus it also adjusts brightness automaticall). I also think the relay circuit only has on/off and no "middle" brightness level.

 

[Nigel Goodwin]

I think the circuit with the resistor, LDR, and LED use the least power just because it has less components.

No, it's a wasteful 'circuit', because it uses MORE current when the LED is dark than when it's lit - it's also not likely to work very well, and not at all unless you can get a really low resistance LDR, most will be too high to work. There's not much to admire about it at all, except it's lack of components - which probably isn't a good thing?.

 

[dknguyen]

Why doesn't the relay circuit cause a short when the relay is closed?

 

[Nigel Goodwin]

Why doesn't the relay circuit cause a short when the relay is closed?

How can it cause a short?, the contacts aren't even shown connected to anything.

 

[bananasiong]

mayb he is saying the NC to GND, NO to Vcc, COM to LED and to GND.

 

[dknguyen]

Short

Well if the relay is closed, doesn't that create a straight path for current to flow down the right branch from +6V -> relay -> transistor ->GND. This would only happen if the gate voltage was pulled high, but since no current will flow through the right branch, I can't figure out whether or not the gate potential will be at GND or +6V to switch the transistor on to cause the short.

Maybe I am misunderstanding the relay symbol?

 

[Nigel Goodwin]

Well if the relay is closed, doesn't that create a straight path for current to flow down the right branch from +6V -> relay -> transistor ->GND. This would only happen if the gate voltage was pulled high, but since no current will flow through the right branch, I can't figure out whether or not the gate potential will be at GND or +6V to switch the transistor on to cause the short.

Maybe I am misunderstanding the relay symbol?

A relay isn't a short circuit, it's a solenoid that moves a mechanical switch, and the solenoid has resistance - which is listed in the specifications of the relay - some can be quite high. But no matter what it's resistance, it's designed to work at it's rated voltage - so why should placing the relay across it's intended supply cause a short?.

 

[Hero999]

https://en.wikipedia.org/wiki/Relay

 

[bananasiong]

can i switch a 3v relay with 5v supply? i do this because i'm worried the power of my battery might decrease. am i going to distroy the relay?

must i connect a diode parallel with the coil of the relay? cathode to Vcc and anode the other end of the coil.