The SparkFun Breakout for SX1509:
https://www.sparkfun.com/products/13601
SparkFun's tutorial:
https://learn.sparkfun.com/tutorials/sx1509-io-expander-breakout-hookup-guide
Datasheet for the SX1509:
https://cdn.sparkfun.com/assets/learn_tutorials/4/3/4/sx150x_789.pdf
Keypad from Adafruit:
https://www.adafruit.com/product/3845
The boards have solder-pad jumpers for configuring the I2C addresses. I left one of the boards alone at 00 for the address 0x3E. On the other board I cut the ADD0 trace to 0 and soldered it to 1. That's 01 for the address 0x3F.
pi@tink:~ $ i2cdetect -y 1
0 1 2 3 4 5 6 7 8 9 a b c d e f
00: -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- 3e 3f
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --
