Adafruit

US$4.24
The buttons screw in place, so you can use with up to ~15mm thick material, and will cover up if the hole if it isn't perfectly round and clean. The action is smooth, without a strong click, yet you can definitely feel when the button is pressed. A tiny micro-switch is pre-installed, with gold plated contacts.
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US$14.00
These 5mm blue LEDs are very bright and have about 20degree LED beam. They go easily into a breadboard and will add that extra zing to your project.
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US$3.42
There are 20 pins, 10 on each side, with 0.1" spacing so you can easily plug it into a breadboard with plenty of space for wiring left over.
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US$4.55
This battery holder connects 3 AA batteries together in series for powering all kinds of projects. We spec'd these out because the box is compact, and 3 AA's add up to about 3.3-4.5V, a very similar range to Lithium Ion/polymer (Li-Ion) batteries, plus it has a nifty on-off switch. That makes them ideal for use with 3.3V projects that have a 2-pin JST connector meant for one of our Li-Ion/Poly batteries. (Of course, you can't recharge them like Li-Ion/Polys, so don't try to plug this into one of our Li-Ion/Poly charger boards!). It also features an ergonomic belt clip for taking your power on the go!
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US$6.92
Keep your power source safe and toasty in these waterproof 2xAA battery holders. They're just like classic switched battery holders, but designed for survival! The case has a rubber gasket around the edge of the case (its black and hard to see in the photos but it is there) and attaches together with 4 machine screws.
Delivery within 2-3 weeks
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US$8.67
Connect this to that without soldering using these handy alligator clip test leads. 20" cables with alligator clip on each end, color coded. You get 10 pieces in 5 colors. Strong and grippy, these always come in handy!
Delivery within 2-3 weeks
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US$5.17
Go 'cold splice' with these snap-able plastic splices for fast connections. Use with any 19-26 AWG solid core wire and slip them into the plastic splice so that they slide into the metal teeth. Then squeeze down with parallel-jaw or slip joint pliers - don't use needle-noses, they have too much angle and not enough force.
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US$9.17
If your receiver/transmitter/transceiver has an MCX connector but you really want to tie it to a BNC antenna, this dongle will do the trick!
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US$26.17
Take advantage of the Raspberry Pi and Beagle Bone's USB port to add a low cost, but high-reliability wireless link.
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US$5.17
 These stickers are a must for anyone making a breadboarded Arduino-compatible. They fit right on top of a DIP ATmega328 (or '168) chip, and clearly indicate every pin as it would be called in the Arduino IDE.
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US$3.42
This cable will let you swap out the stock 150mm long flex cable from a Raspberry Pi Camera (either 'classic' or 'NoIR' type) for a different size.
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US$6.92
These particular 3.5mm terminal blocks are our favorite: big enough for a range of wire gauges, easy to adjust with a screwdriver, and not too chunky. The only down-side is that they do not fit into 2.5mm spaced holes (they're 3.5mm spaced) - but they do fit in most perf/proto-board if you rotate them diagonally.
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US$11.46
These buttons are bigger than the 6mm ones we carry, and come in a pack of 15. Each button comes with a candy-colored square cap that snaps on. You get three of each color-- blue, grey, yellow, green, and red. The square plastic button tops affix easily to the tactile switch body.
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US$5.25
This is a 28 pin ZIF socket designed for programming DIP packages of integrated circuit of up to 28 pins and 0.3" wide chips.
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US$1.74
Silicone-sheathing wire is super-flexible and soft, and its also strong! Able to handle up to 200°C and up to 600V, it will do when PVC covered wire wimps out.
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