Here's a useful circuit, he is a TTL square wave oscillator, it uses three logical ports, and also a capacitor and a resistor to adjust the frequency. The circuit can be used as a Clock Oscillator, standard signal, etc. The oscillator uses a 7400 integrated circuit, the table below shows the values of C * and R * to achieve the desired frequency, which can be chosen between 100 hz and 1 MHz
SOURCE: http://www.eletronica.com/circuito-gerador-de-onda-quadrada-ttl/
