Schematic of the HC-SR04 tester module . It’s undoubtedly a nice concept and clever design. The compact module helps us to test HC-SR04 modules quickly without a costly microcontroller setup (no code ...

[Emil] reverse engineered the schematic of the HC-SR04 and found some interesting design decisions. A Max232 RS-232 converter chip is used for its +-12V +-10V charge pumps.