Note: added some more photos for clarification.

https://hackaday.com/2022/12/11/a-straightforward-old-fashioned-dac/

1. Schematic

The assembly consists of two CD4007UBE, working as inverters/switches (switches are one inherent feature of R-2R ladder DACs). 

CD4007A is given in my book source, which has lower max voltage but is the faster version (30ns vs 20ns; one has to be aware of the general rule not using a faster logic family than necessary because of noise problems). 2R = 100k +/- 1%.

2. Programming

MicroPython program for an ad-hoc test, generating digital output on a Pi Pico (ought to generate 2.5V): 

from machine import Pin
lsb = Pin(7, Pin.OUT) # bit0, least significant bit
bit1 = Pin(8, Pin.OUT)     
bit2 = Pin(9, Pin.OUT)     
bit3 = Pin(10, Pin.OUT) 
bit4 = Pin(11, Pin.OUT) 
msb = Pin(12, Pin.OUT) # bit5, most significant bit (2^5 = 32)
lsb.high()
bit1.high()      
bit2.high()      
bit3.high()
bit4.low()      
msb.low()

For more sophisticated applications one can deploy the RP2040 Single-cycle IO block (SIO).

C program would look something like (also ought to generate 2.5V, 001111 is 15d):

(...)
void setOutput(int DAC)
{
  *(SIO + 0x014 / 4) = DAC << 7; // we start on GP7
}
(...)
setOutput(15ul);
(...)

3. Using an I2C bus I/O expansion

Expending six Pico GPs (or more) may be rather impractical for some projects. 

The I2C bus would need only two connections, so one can go with an additional I2C IC, such as the MCP23016 I/O expansion.

A code example for a simple sawtooth generator is uploaded to my github repo.

4. Applications

SAR ADC, function generator, sound generator, etc.

5. Costs

I paid 50 Cents for each IC, and a few Cents for the resistors. MCP23016 was about 2€.