Serial communication with Arduino using a 7404

7404-rs232Serial communication with a DIY Arduino remains a topic of interest as the USB connection with an FTDI chip is not always easy to DIY.
A MAX232 as shown before can do the job and so can a few discrete components. But if you have 2 gates left in e.g. a 7404 (a HEX inverter) you can use that one too:

If you have 3 gates left, you could add one to the above circuit to transfer the DTR signal of pin 4 (on a DB9) to use that as a Reset for the DIY Arduino. Just make sure you feed that signal to the input of a spare gate via a voltage divider (2x10K).

Oh well, let me do that for you. It would look like this:
rs232-ttl

Simple Arduino with 3.3 Volt

The Simple Arduino published earlier can easily be outfitted with a 3.3 Volt outlet, as there is space enough on the stripboard
5and3-3volt
Adding 3.3 Volts is easily done with a 3.3 V regulator for which I have chosen the lowdrop TS2950C33. This regulator can deliver 150 mA which should be enough for most applications.

The circuit is quite straight forward, just use the input of the 5 Volt regulated power supply of the board as a feed for the TS2950. If you pick another 3.3 Volt regulator, make sure that 5V input is enough, otherwise you may need to feed it from the unregulated power line coming from Diode D1. The capacitor in the 3.3 V line does not need to be that big: 1uF even 220nF is probably enough already, but I just happened to have a 10uF.

It is very easy to find space in the setup as originally proposed by Iñigo Zuluaga, as shown here:

It requires only 2 extra wires, a TS2950C33, a capacitor and of course a one pin larger connector and it nicely fills the ‘empty space in the upper right corner.

The Stripboard looks like this: It requires cutting a strip and running 2 wires. (indicated in red). If you wish, you could extend the header with even 1 more hole to not only include the 3.3 V pin, but run an extra Ground right next to it. The ground is already running on that strip
arduino-w-3-3V

Mind you, it is the stripboard as seen from the copperside, thus looking at the under side of the physically not visible components (an X-Ray view so to speak).
P1040272e

Simple SD card interface for Arduino

In an earlier circuit I showed a simple way of  adding an SD card to an Arduino. That works well. Sadly because the Arduino is 5 Volts and the SD card needs 3,3 volt levels, a level adapter was used, consisting of resistors.
As said, it works well, but it is not really an elegant solution. It is better to use another chip (The 74HC125)  to do the level shifting, as shown in the figure below:
SD card2

To keep the circuit a bit simple, I have just drawn the buffer gates of this chip, but there are a couple of more pins that need to be connected: Pin 1,4, 7 and 10 Need to be connected to ground. Pin 14 (Vcc) needs to be connected to 3.3 V.

The reason why so many of the pins need to go to ground is because Pin 7 is the regular ground of the chip, but pin 1,4 and 10 are the negative control gates of the buffers that are used. The 74HC125 is a Quad bus buffer so you will have one buffer to spare. It might be wise to connect the input of that buffer (pin 12)  and it’s negative gate (pin 13) to 3.3 V as well to avoid unpredictable behaviour.