# 1.5v LED Fuse Bulb?



## jbrett14 (Nov 12, 2012)

Anyone know where I can get a 1.5 volt LED "fuse bulb"? 

The one I have for my continuity tester is an incandescent bulb which looks exactly like the little glass fuses that are approx. 1" long. I would like to replace it with an LED type but can't find anything with that low of a voltage rating.


----------



## uk_caver (Nov 13, 2012)

I think you'll have difficulties since 1.5V isn't really enough to generate photons of visible light in an LED (ie an LED which ran off 1.5V would be producing infrared light - it's just the way the physics works).

If you really wanted to have an LED, you'd either need to change the voltage source for something higher or add some electronics to boost the voltage to a suitable level to light an LED (which isn't hard if you have space in the tester to house suitable electronics).


----------



## Gunner12 (Nov 13, 2012)

I think a red LED would light up dimly at 1.5v, so you could try that.

A simple boost circuit and a LED could work as well, depending on how much current he power source can provide. I'm thinking a joule thief could work.


----------



## uk_caver (Nov 13, 2012)

Some lighting on a full alkaline cell might happen (primary lithium cell might be better choice)

However, one characteristic of a bulb is that there'll be a resistance below which it won't meaningfully light up, or will at least light dimly and more orange or red than normal.

An LED won't necessarily give comparable behaviour, with colour being possible even at low currents (high resistances), so a lot could depend what level of 'continuity' the tester should be giving yes/no/maybe results on.


----------



## Lynx_Arc (Nov 13, 2012)

Like others have said forget trying to light up an LED off a 1.5v battery source without a boost circuit, you either need one or change the power source to a 3v or higher amount like a lithium or lithium rechargeable format or double up on alkalines somehow. A 1.5v battery unless it is a lithium primary may start out perhaps 1.6v at the most but half way through its life drop to 1.3v which means whatever dismal light you could even fathom seeing would be even dimmer.


----------



## jbrett14 (Nov 13, 2012)

I appreciate the input guys, but I am a bit confused. Are you guys saying that even a cheap $3 1 x aaa LED flashlight has a "boost circuit"? I just assumed, given the low cost, that these were as basic as a battery, a switch, and an LED, with a housing.


----------



## moderator007 (Nov 13, 2012)

jbrett14 said:


> I appreciate the input guys, but I am a bit confused. Are you guys saying that even a cheap $3 1 x aaa LED flashlight has a "boost circuit"? I just assumed, given the low cost, that these were as basic as a battery, a switch, and an LED, with a housing.


A 1AAA flashlight would most definitely have a boost circuit. A 3AAA flashlight on the other hand may or may not have a circuit. Most of those really cheap 3AAA lights are direct drive. There's really not much to making a cheap low current boost driver.


----------



## ianfield (Nov 15, 2012)

Lynx_Arc said:


> Like others have said forget trying to light up an LED off a 1.5v battery source without a boost circuit, you either need one or change the power source to a 3v or higher amount like a lithium or lithium rechargeable format or double up on alkalines somehow. A 1.5v battery unless it is a lithium primary may start out perhaps 1.6v at the most but half way through its life drop to 1.3v which means whatever dismal light you could even fathom seeing would be even dimmer.



Its common for keyring LED lights with a single CR2032 for a cloured LED or 2x CR2016 for blue or white, these rely on cell internal resistance which doesn't do the cell or LED much good!

A simple blocking oscillator/inverter can be made that drives any LED from a single alkaline cell. The transformer is dead easy to wind with about 30 - 40 turns cantre tapped. You can use a short bit of ferrite rod, but I'd reccomend a toroidal ferrite from an old motherboard - you could also use a common mode choke from the input stage of a PSU, just connect the finish of one winding to the start of the other (that's your centre tap). One end to the C of an NPN transistor, the other end; via a 1k preset resistor to B - the E terminal of the transistor to -ve & centre tap to +ve 1.5V.

If you have trouble following my description, there's plenty of blocking oscillator schematics on the web - some shown driving a LED.


----------



## Lynx_Arc (Nov 15, 2012)

ianfield said:


> Its common for keyring LED lights with a single CR2032 for a cloured LED or 2x CR2016 for blue or white, these rely on cell internal resistance which doesn't do the cell or LED much good!
> 
> A simple blocking oscillator/inverter can be made that drives any LED from a single alkaline cell. The transformer is dead easy to wind with about 30 - 40 turns cantre tapped. You can use a short bit of ferrite rod, but I'd reccomend a toroidal ferrite from an old motherboard - you could also use a common mode choke from the input stage of a PSU, just connect the finish of one winding to the start of the other (that's your centre tap). One end to the C of an NPN transistor, the other end; via a 1k preset resistor to B - the E terminal of the transistor to -ve & centre tap to +ve 1.5V.
> 
> If you have trouble following my description, there's plenty of blocking oscillator schematics on the web - some shown driving a LED.


I am not sure why you are addressing me with this post, I covered the CR2032 with my "3v or higher amount like a lithium or lithium rechargeable" comment and your "simple blocking oscillator/inverter" is a BOOST circuit. I've built boost circuits but would rather get them premade as winding coils and getting parts to match up properly is a hassle and can cost more than going to walmart and buying a 1AAA rayovac led light for $3 and scavenging the driver out of it.


----------



## ianfield (Nov 15, 2012)

Lynx_Arc said:


> I am not sure why you are addressing me with this post, I covered the CR2032 with my "3v or higher amount like a lithium or lithium rechargeable" comment and your "simple blocking oscillator/inverter" is a BOOST circuit. I've built boost circuits but would rather get them premade as winding coils and getting parts to match up properly is a hassle and can cost more than going to walmart and buying a 1AAA rayovac led light for $3 and scavenging the driver out of it.



Its pretty much implied in what I said that salvaged materials are the order of the day - only cost is electricity bill to run the soldering iron.


----------



## Lynx_Arc (Nov 15, 2012)

ianfield said:


> Its pretty much implied in what I said that salvaged materials are the order of the day - only cost is electricity bill to run the soldering iron.


I'm of the opinion the OP isn't interested in making one from parts perhaps not even able to do so without much assistance. It would have been easy to query a search engine with the words "1.5v white LED" and figured out what we have been saying in less than a minute of reading.


----------



## icecube (Nov 16, 2012)

I'll garnar that the "old" lightbulb can probably be replaced with a 5mm white LED. 

5mm white LEDs have a typical forward voltage around 3.4v-ish, depending on a lot of things. At that point they are fairly bright although they can top out around 3.6V. 

You can get something like this which should work just fine. You may want to look at a diffused LED or make a diffuser because white LEDs driven at that voltage (5mm common "Water Clear") can be very bright. I'm not sure what your individual preferences are but red is easier on the eyes. On the other hand if you did want to use red with that little boost circuit you'll need a resistor to throttle the input voltage.

Typical current draw for such an LED is around 20mA, although that can be extended to 30mA for special applications which I won't go into.


----------

