I noticed I was doing a lot of breadboarding at my desk (as opposed to my worktable in the basement). That was fine, but I always had to have a battery pack/power supply hanging off of the breadboard. I had this neat enclosure I got as a sample that has a battery compartment. It’s made for AA batteries, but two 9v batteries worked fine (I put electrical tape over the other contacts). I built two power supplies (5v and 3v) on a project board and mounted it inside.
I soldered some leads to the underside of the breadboards, then wired up some switches and power LEDs
I made the 5v (green) LED brighter because I wanted to be sure I noticed that there was a higher, potentially damaging voltage on one of the power rails. I put a few other safety features in place, such as diodes between the positive terminals of the 9v batteries (in case I put one in backwards), diodes on the ground rails (to prevent unwanted circuits through the power supplies), and disconnection of the power rail(s) through the switch(es) to completely isolate them when the switch is off.
Here’s the schematic:
Let me know of any errors in the comments.
Nice work! The PCB that you made, is it the old offset one or a new one?
It’s just a project board. I bought a few of them a while ago on eBay. You can see where I used jumpers to connect sets of holes. These boards have a pretty good layout, with center power/ground rails, and good groupings of holes/pads. They even have some nice pads you can use for SMD spaced parts. Only problem I run into is that only the outside holes are large enough for things like the v-reg leads and larger hookup wire. That’s why I had to jumper everything to the edge of the board (looks better anyway).
A quick glance at your schematic tells me that D3 and D4 is the wrong way around, also you should note that these 2 will cause the output voltage to be 0,6V lower than what the regulator puts out :)
Also batteries run out all the time and can be expensive in the long run for testing, why not use a wall-wart?
Ooops! They sure are backwards! I’ll try to get a corrected schematic up soon.
I realize there is a voltage drop; I get almost exactly 3v and 5v at the board. Works fine for the MSP430s and other ICs I usually use.
I agree that batteries are expensive, and I may add a jack for a wall-wart at some point. The initial plan, however, was portability and to be self-contained. In the post, I mentioned always having to have a power supply hanging off the breadboard; would a wall-wart be so different? At any rate, the circuits I use consume little power (probably lose more in the V-regs), and the power is not on for long periods, so the batteries should last a long time.
Thanks for the correction on the schematic!
I guess you have a good point about the wire hanging off the breadboard.
And with regard to power consumption yeah we are talking about 30% output compared to input for the 3V reg at best. Little better for the 5V reg since it doesn’t have to drop quite as big a voltage. The very inefficient linear reg was also the reason I didn’t really like the battery :)
I’m not too worried about the inefficiency. The power is only turned on for a few seconds to a few minutes at a time. The batteries should last a bit.
The voltage drop on the diodes, however, is causing problems. I’m probably just going to pull them and replace with jumpers.
Why not use something like an LM317 in place of the 78XXs? You should be able to tune those to just a bit higher than 3V/5V, to account for the diodes’ drop.
g: LM317s probably would have been better for this. I’m just getting started with electronics, and I just went for what I thought was the simple solution. This just shows why invention is tough; between idea and implementation stand the details. ;-)