DSO211 Handheld Digital Oscilloscope for US$3.88 on eBay
https://www.ebay.com/itm/Portable-Mini-DSO211-Pocket-sized-Handheld-Digital-Storage-Oscilloscope-LOT-LU/162736692611?_trkparms=aid=222007&algo=SIM.MBE&ao=1&asc=20160908110712&meid=7e5413e778df400bb1eb1978259b31b7&pid=100677&rk=4&rkt=30&sd=332253980766&itm=162736692611&_trksid=p2385738.c100677.m4598
https://www.ebay.com/itm/Portable-Mini-DSO211-Pocket-sized-Handheld-Digital-Storage-Oscilloscope-LOT-LU/162736692611?_trkparms=aid=222007&algo=SIM.MBE&ao=1&asc=20160908110712&meid=7e5413e778df400bb1eb1978259b31b7&pid=100677&rk=4&rkt=30&sd=332253980766&itm=162736692611&_trksid=p2385738.c100677.m4598