If you ever built a line following robot, you’ll be nostalgic about [Jeremy’s] light-seeking robot. It is a very simple build since there is no CPU and, therefore, also no software.

The trick, of course, is a pair of photo-sensitive resistors. A pair of motors turns the robot until one of the sensors detects light, then moves it forward.

This is a classic beginner project made even easier with a 3D printer and PCB to hold the components. You might consider using an adjustable resistor to let you tune the sensitivity more easily. In addition, we’ve found that black tubes around the light sensors in this sort of application give you a better directional reading, which can help.

The robot only has two wheels, but a third skid holds the thing up. A freely-rotating wheel might work better, but for a simple demonstration like this, the skid plate is perfectly fine.

This is a good reminder that not every project has to be fantastically complex or require an RTOS and high-speed multi-core CPUs. You can do a lot with just a handful of simple components.

If you want to follow a line, the basic idea is usually the same, with perhaps some different sensors. Usually, but not always.

[Anthony Francis-Jones], like us, has a soft spot for the educational electronic kits from days gone by. In a recent video you can see below, he shows the insides of a Philips EE08 two-transistor radio kit. This is the same kit he built a few months ago (see the second video, below).

Electronics sure look different these days. No surface mount here or even printed circuit boards. The kit had paper cards to guide the construction since the kit could be made into different circuits.

The first few minutes of the video recap how AM modulation works. If you skip to about the ten-minute mark, you can see the classic instruction books for the EE08 and EE20 kits (download a copy in your favorite language), which were very educational.

There were several radios in the manual, but the one [Anthony] covers is the two-transistor version with a PNP transistor as a reflex receiver with a diode detector with a second transistor as an audio power amplifier.

We covered [Anthony’s] original build a few months ago, but we liked the deep dive into how it works. We miss kits like these. And P-Boxes, too.

It used to be a rite of passage to be able to do the math necessary to design various bipolar transistor amplifier configurations. This doesn’t come up as often as it used to, but it is still a good skill to have, and [Void Electronics] walks us through a common emitter amplifier in a recent video you can see below.

The input design parameters are the gain and the collector voltage. You also have to pick a reasonable collector current within the range for your proposed device that provides enough power to the load. You also pick a quiescent voltage which, if you don’t have a good reason for picking a different value, will usually be half the supply voltage.

The calculations are approximate since the base-emitter voltage drop will vary by temperature, among other things. But, of course, real resistors won’t have the exact values you want, or even the exact value marked on them, so you need a little flexibility, anyway.

There are other ways to approach the design. But most design guides will make the same assumptions: Ic=Ie, base current is negligible, and similar simplifying assumptions.

At the end, the circuit winds up on a breadboard so you can see how close the predicted performance is to the design.

We’ve covered biasing bipolar devices a number of times. We’ve even modeled circuit variations in a spreadsheet.