Basically, Yantras are the sources of energy and it will be activated if we recite a certain mantra constantly. Once it is activated, it will start transmitting its powers to the surroundings. It makes a impact to a human based on his capacity. It is again proportional to his spiritual capacity where he can intake the power. It is a vast subject to get a complete knowledge of Yantra.
Worshipping God/Goddess using Yantra is one of the best way to attain salvation. It is strictly adhered in all temples. We can keep some Yantras in home as well for worshiping under some restrictions.
The most popular yantras for worshiping in home are Shri Yantra(Yantra Raja), Vasthu Yantra, Navagraha Yantra. Now a days people started having yantras of Grama devate, goddess Mahalakshmi, Ganesha, etc.
Proper worshiping of Yantra will be remedy for get rid of all planetary doshas. Its ultimate goal is the person should attain enlightenment.
It is not recommended to have yantra as pendant/ring in one's body. Many people are now a days misleading our spiritual and religious values by introducing such practices. One should keep only ShriYantra with restrictions as same as Rudraksha.
In Shivism, Rudraksha is the symbol of Lord Shiva. In Shaktam, Shri Yantra is the symbol of Goddess Lalitha Tripura Sundari. If one knows about Shri Yantra, we come to know that it is a form of Lord Shiva and Shakthi.
To get rid of any kind of doshas, Worshiping Shri Yantra is the best remedy. It not only resolves the problem also tunes the life of the person to attain the righteous path.
I will add more details about Shri Yantra in personal blog soon. Stay tuned..